I thought I'd share my recent experience (last weekend) with acquiring the National Rail travel card and using the 2 for 1 vouchers.
We bought our off peak travel cards from Waterloo train station for 7 pounds each. It took about 30 seconds to spot a very large window display advertising the 2 for 1 offers in the National Rail ticket office. We simply asked the cashier for the appropriate zone 1-2 one day travel card so we can use the 2 for 1’s. He swiped our U.S. (chip-less) credit card and we were off. It was that easy.
Using the 2 for 1’s was also very easy. At the London Eye they asked for and checked that we had the appropriate travel cards. Two restaurants in Covent Garden didn't check our travel card. They only wanted the voucher. We made sure to also tip on our free meal! It was that easy. So simple. We saved over 40 pounds in one day.