Prepurchase train ticket from CDG to Gare de Montparnasse

This is a local Paris Transport ticket. You won't get anywhere with a OUI app as that is for SNCF national trains. It probably showed you the price for traveling to a distant location then back to Paris to Montparnasse station. Just buy a regular ticket to Paris on the RER train and it will include the connection via Metro to Montparnasse.

There is also a bus from CDG to Montparnasse. You might be able to get a prepaid ticket on that.

Since there are 2 people, you might also consider a taxi, which is a fixed price to Left Bank locations.

The bus to Montparnasse doesn’t exist anymore.

Thanks for that info, its the RER B train/Metro combination I'm hoping to prebook, I read you could get tickets for this in advance

The ticket from CDG to downtown Paris is 10.30 Euros (so 20.60 total for 2 people one way). The amount of time to buy the ticket from an automated machine is about 2 minutes (English menus, credit card, and choosing downtown Paris). There is also an area with English speaking clerks that can sell you the same tickets and answer your questions (hit or miss).
Considering the variability of the plane's arrival, customs/immigration/baggage claim, the long walk, and potential other delays, it really is not that likely that the 2 minute savings pre-buying will matter. I'd just buy the tickets on the way to the RER platform (literally right outside the platform area) and not worry about it.
And the ticket for the RER includes a transfer to the metro or a bus. One ticket to get to your destination.

The fixed-rate fare for a taxi from CDG to the Left Bank (I.e. Montparnasse) is 58€ for up to four people.

I can’t think how one would pre-purchase an RER ticket. Maybe it’s possible (I’m not sure), but I don’t have any idea.

There is no way to prepurchase that ticket (without diving into things only local commuters have access to), and there is no need at all. Ticket machines take cards and cash, are very easy to use, and plentiful. I haven't done it in several years as I now take taxis (thanks to the flat fare mentioned above), but I never found any significant line at the ticket machines when I took that RER.

Also, the ticket will include the metro fare, so make sure to keep it until the very end.

Paris airport website says RER fare is 11.40 EUR.
https://www.parisaeroport.fr/en/passengers/access/paris-charles-de-gaulle/public-transport/rer-b
I believe you can also buy RER t+ tickets at the Paris Information desks at arrivals in Terminals 2E and 2F.
https://www.visitparisregion.com/en/services/tourist-information-centres/pit-aeroport-paris-charles-de-gaulle

So how long did you give yourself between the plane landing and your train departure. That might be a relevant piece of information to discuss how you get from point a to point B.

I’ve encountered long lines and 15+ min waits to use the RER ticket machines at CDG, and an hour + long wait many times for the ticket window with a real person. There’s no way around it though, and hopefully the day you’re there the wait will be short.

To emphasize: Hold onto that ticket. You will need it to exit the RER and then enter the Metro. Nothing to do at the Metro exit except to discard the ticket outside the exit gate (there are waste bins). The authorities do fairly stern spot checks in the Metro.

From the OP's other thread, looks like departing August 7, arriving at CDG on August 8 (my assumption), and traveling toward the Dordogne on August 9. So it does not seem there is any rush to get to Montparnasse neighborhood to travel following day.

I have just under 4 hours from flight touchdown at CDG till the train departure for Bordeaux.
Should be plenty unless there's a combination of a flight delay + long immigrations queues, then a 2 minute wait versus a 20 minute wait at a ticket booth could make a difference.

Well I personally think it’s a little optimistic. I’ll give myself 4 hours for non protected connections at the airport.

And honestly I’m not sure which is quicker metro or a taxi, but you might want to consider the taxi simply because it will take you directly to the train station. The way you’re going you’re going to get to the metro station and at that particular location it’s been quite a trek from where you get off the metro to where you catch the train

And that 15 minute walk from Defert Rochereau you mentioned in your original post is a little on the optimistic side. I’ve done it quite often and I walk pretty fast and you really have to be moving quickly to do it in 15 minutes. hauling luggage? I don’t know

It sounds like you already purchased nonrefundable tickets from Montparnasse to Bordeaux. I think a taxi will be your best option. 2 people will be 23 EUR with the RER/Metro, so spend another 35 EUR on a taxi. It eliminates so many worries. Navigating the RER and Metro (for the first time?) in unfamiliar surroundings. Taxi will take you and your luggage from the airport to the front door of Gare Montparnasse. There are times to be frugal on vacation and times to open up the wallet. This is one of those times to spend a little more and get things off on the right foot.

One of the sites said its possible to pre purchase the ticket for this, as I might be in a hurry if my plane is delayed I'd like to do this, but I can't find any information on where/how to do this. Is it possible to prepurchase a couple of tickets and put them on an iPhone?

I have a bad feeling about this. Why would you be in a hurry to catch a commuter train that runs several times an hour? Are you trying to catch a long-distance train at Gare de Montparnasse? If so, I wish you luck. Unless you bought a combined air/rail trip such as those offered by Air France then if you miss the train then it's your bad luck and you need to buy a new train ticket. (edit: I read back and see that's what you're doing) 4 hours is usually a safe window, but there's another risk. In the paper they've reported on airlines cancelling flights and changing their schedules. You don't say when you're travelling, if its next summer maybe things will have settled down by then, but if it is imminent travel then airline churn may be a potential problem.

Anyway, there is a secret way to buy a RER ticket with no waiting. Simply go to the TI office in the arrival terminal. The train station terminal has long lines for the staffed windows. But the TI office in CDG sells RER tickets as well as Metro tickets if you're staying in Paris. They speak English and take US credit cards. See https://bonjourlafrance.com/attractions/france/ile-de-france/paris/information/paris-cdg-airport-terminal-1-tourist-information/ Check to see if they are still open due to covid

If you are arriving in August, the taxi, at 58€ will likely be the fastest option and certainly the easiest.

If you are arriving in September or later, the RER B/métro 6 from Denfert Rochereau may be faster. I have taken too many 90 minute taxi rides from CDG to addresses near Montparnasse. If it´s raining, definitely take the train.

Paris airport website says RER fare is 11.40 EUR.

That might be the case if you purchased separate RER/métro tickets but one ticket costing 10.30€ is all you need. The best place to obtain information about ticket pricing for public transportation is from the RATP website: https://www.ratp.fr/

I’ve encountered long lines and 15+ min waits to use the RER ticket machines at CDG, and an hour + long wait many times for the ticket window with a real person. There’s no way around it though,

I have never seen waits as long as 15 minutes at the SNCF ticket machines at CDG (there are several banks of machines) but if you do not want to wait, take the free train, CDGVAL, to Roissypol, purchase and board the RER B into Paris at Roissypol (between terminals 1 and 2).

I have just under 4 hours from flight touchdown at CDG till the train departure for Bordeaux.

Should be sufficient time unless you arrive CDG late.

Why would you be in a hurry to catch a commuter train that runs several times an hour?

The RER B into Paris runs every 8 to 10 minutes during the day and purchasing tickets and boarding the next train should never take long.

I would also check here to make sure there are no planned outages on the RER on your day

https://www.ratp.fr/en/infos-trafic/rer/b#container

I was just reading some website and it said a secret way to skip queues is to go to the Tourist Information desk in T1 as they sell RER/Metro tickets.

Does anybody know if there is a TI desk in T2 that does likewise?

I posted this link on this thread 3 days ago.

I believe you can also buy RER t+ tickets at the Paris Information desks at arrivals in Terminals 2E and 2F.
https://www.visitparisregion.com/en/services/tourist-information-centres/pit-aeroport-paris-charles-de-gaulle