London 2for1 vs London pass

Depends where you buy the train tickets, but it can be very fast. I once used it by buying at West Brompton that took all of one minute. If you're getting your tickets at a major station though it could take longer.

Hi akkejakke, thanks for your reply. I am asking the attraction tickets, such as London tower etc, Do I have to line up each main attraction ticket window to get the 2for1 discount ticket?

I just wanted to clarify that you are working off of current information about 2 for 1 offers. I know that there was a change in the past year or so where some of the tickets or passes that formerly would make one eligible for these offers no longer worked.

https://www.daysoutguide.co.uk/2for1-london

I cut and pasted this from the National Rail website about which tickets work for these promotions and which ones do not.
Is my ticket valid for 2FOR1?
YES! The following ticket types are valid for 2FOR1 and other offers on daysoutguide.co.uk:

National Rail orange paper tickets including:
Advance
Anytime
Off-Peak
Super Off-Peak
Travelcards - purchased from a National Rail station only
Print @ Home or Mobile tickets (m-Tickets) for travel on National Rail services (showing the National Rail symbol)
Carnet Tickets
Season tickets
Annual Gold Cards
Gold Record Cards (when an Annual Season ticket is held on smartcard)
Train company issued smartcards (e.g. The Key, c2c Smart)
NO! The following ticket types are cannot be used for 2FOR1 and other offers on daysoutguide.co.uk:

Oyster Cards
Transport for London (TfL) issued Travelcards, including Travelcards bought on visitorshop.tfl.gov.uk
Freedom Passes
Contactless payment cards or mobile payments
Heathrow Express and Heathrow Connect tickets
Stansted Express and Gatwick Express in-flight or mobile tickets where the National Rail logo is not shown
BritRail Passes
Eurostar tickets
MegaTrain tickets
If you are making a one way journey or your ticket is valid for one day, you can use your National Rail tickets and offer vouchers only on the day of travel.

If you are making a return journey, you can use your National Rail tickets and offer vouchers any day on or between your outward and return journeys. For example, if you arrived on Thursday and will return on Sunday, your tickets will be considered valid for the offers on Thursday, Friday, Saturday and Sunday

Now, looking at the rest of your question. There are some places where the London Pass would save you time because you wouldn't be going to a ticket office/dealing with vouchers etc., and some places where it would not.
Examples:
Tower of London: It should save you time as you just show your London Pass off of your phone, go through security and you are in. I think you would need a stop at the ticket office with the 2 for 1 offer.
Westminster Abbey: No time saving. Everyone stands in line whether they have advance tickets or not. The London Pass might save you a minute at the cashier, but that would be about it.

As with all things London Pass, it really depends on the sites that you want to go to and then pricing everything out. Sometimes it is money/time saving and sometimes it is not. I do find that it is convenient and entry is fairly quick. The other advantage is that one can be fairly spontaneous and change schedule a little easier.