Getting from Heathrow to Manchester

May 26 is a public holiday. It is affecting the trains to Manchester in that no services can run on the direct route due to engineering work (they are being diverted via Birmingham with an extended journey time).
Also no services are running to Scotland via Preston due to separate engineering work so are further diverted non stop via Manchester.

I don't understand that you have both Brit-rail and Two Together. You don't need both as Britrail covers all trains, and there is no further Two Together Discount for Britrail.

Assuming you are travelling with Britrail you don't need tickets and just take whatever is the next train when you get to Euston.

If you are needing a ticket (not travelling on Britrail) Advance train specific tickets are on sale now starting at £43.50 (before railcard discounts). But yes it would be a total guesstimate against the flight being retimed/delayed/or even running early as eastbound TA's often do.
Far better to buy an Off Peak Single for £79.80 each (before railcard discounts) which is valid on any train on a walk up basis.

Buy on the Avanti website. Split saver ticketing is utterly crazy when travelling into the unknown like this. In any case the usual split fare hack on Advance Fares which gets you to Manchester from London for £10 after railcard discount doesn't work with the diversions.
The other usual split fare hack of St Pancras to Sheffield to Manchester is working that day (about £18 that way after discounts with careful choice of train) but is similarly not adviseable.
I can see a split fare offer that day on two Advance Tickets of a 5 minute change at Stafford which feels like loony tunes to me.

In theory with everything in your favour time from landing to Euston should be two hours. The Piccadilly Line tube route is closed that day from Hammersmith through central London to Cockfosters- so you are on Heathrow Express or the Elizabeth Line changing en route for Euston.

The fastest and cheapest route is Heathrow Express (Early Bird Tickets for £6.65 to Paddington available with your railcard + the tube fare of £2.80)

Thanks so much for the detailed reply!
We are NOT using BritRail for the London to Manchester leg - that's why the Rail Card.
The timing of everything is what's throwing me off - will most likely go w/ what you recommend instead of trying for Advance tix & split saver - the holiday definitely throws a wrench into things....

We are NOT using BritRail for the London to Manchester leg - that's why the Rail Card.

Doesn’t the Britrail pass cover Euston to Manchester?

Yes BritRail covers it but we bought the group pass for after meeting rest of party in Manchester

Where are you flying from??? United no longer flies direct to Manchester from NYC (my area), but they do from other cities, like Chicago. The early reply you got suggests that it might be smarter to fly directly to Manchester, even if you have to change once in the US or in Europe. Have you priced such a trip?

Sorry, I didn't really go into our entire trip - flying into Heathrow as we are planning 3 days in London on back-end of the trip

You could do an open-jaw flight into MAN and back from London. There are a few direct flights from the US into MAN (JFK, ATL, MCO, LAS) and many one stop options via DUB, AMS, CDG etc. Or you could of course fly from LHR to MAN, although that might be 3xoensive on that particular day by now.

Normally, I would say fly from Heathrow to Macnhester. But, since it's a holiday, I looked at the fares for a flight that day and they are ridiculous unless you want to hang around the airport all day.

We already have tickets but will look into what y'all have suggested

Quite apart from the fares LHR to Manchester the flights are at 0645, 0855, 1500, 1745, 2035 and 2140. The 1745 being the first sensibly priced one (still more than the train)

The 0855 would be a big gamble if on a separate ticket. Even on a same ticket if the transatlantic flight was late that is a long wait- because the demand for flights for two such close cities collapsed 30 years or more ago when the fast, 3 trains an hour service from Manchester to London started.

Plus some totally crazy connecting routings via Belfast, Aberdeen or Inverness. No sane person would take over 6 hours to fly from London to Manchester via Inverness even if the fare wasn't £400.