2 for 1 tickets

Yes, but only for that day of travel, unless you have another ticket out of London later. On TripAdvisor in the London forum, people discuss buying a cheap “decoy ticket” from London for a date later in your stay to qualify for the 2 for 1 deals the entire time, but we have never done that. We like the paper 7-day travelcard if we want to do lots of the 2 for 1 deals.

The gang running the promotion for the railways say in their FAQs that they are on to decoys and attractions will often disallow them.

The idea is that you take the train to the attraction, so you need one to the part of London you will be visiting, for same day usage, or a ticket to and then later from, which allows you use of the scheme for the duration of time between arrival and departure.

If you use the "Out" part of your ticket to Windsor, you're not travelling to any London attraction. If you use the "Return" part many places will disallow it because you aren't going to the place, you are returning. But as Lola says many will allow it, just for the date on the ticket.

Worth mentioning (maybe) that the 2FOR1 offers are not just for London they are available on a number of attractions throughout the country travelled to by rail. Nothing particularly in the Windsor area though, apart from Legoland.