If I purchase a one-way rail ticket from Dover to London, and purchase two 7 day travel cards, can my wife and I use the 2-for-1 vouchers during the 7 day period starting with my train arrival? Or, must I have a return trip ticket to Dover?
By themselves, buying two seven-day paper travelcards at a National Rail station (not a Tube station) allows you to use the 2-for-1 vouchers for all seven days that your travelcards are valid. Unless you want your seven days to start the day after you arrive in London, tickets from Dover to London don't need to play a part at all.