After using British rail passes for many years, I bought the Two-Together for the first time to discover that it may be the most efficient and cheapest way for 2 people. This is how to get it and use it:
1- The online application works if you have an address in England. Because my address is in the US, I downloaded the paper document, and took it to the first ticket office when I arrived in England. You need a passport-size picture for the application. They issue your ID on the spot for 30 pounds. You get one ID for the 2 people. A replacement card can be done if you keep your receipt and if needed.
2- The ID can be used for buying paper or digital tickets. Make sure you mark the pass on the options or you pay the full fare. This pass can be used in combination with other discounts. In my case, for example, I bought a Discoverer Card for unlimited travel in the Cotswolds. I added the Two-Together and the savings were even better.
3- Carry the card on you on the train. It will be inspected with your discounted tickets. No card, you pay the difference in fare.
The tickets may be inspected more than once in longer rides. Just keep it handy.
4- Gatwick Express accepts the pass. The advertised one-way is 19.90 pounds per person. With the card, I paid 23 pounds for 2, buying directly on-line from Gatwick Express, digital tickets.
5- If you want to improve your savings, read more on splitting tickets in England. In some cases, it is way cheaper to buy 2 tickets, breaking your travel in 2 segments. I tested this option more than once, and the savings were of over 60% in both cases.
If you forget to get the pictures, there are ID booths everywhere in major stations. Remember that ticket pricing changes everyday so you may be better off buying 2-3 days before you travel. Some routes have SuperSaver fares well in-advance too. Have a great trip!
Note: Some great fares are valid for the train you select only. That is, you cannot change trains or departure times. If you do, your ticket is not valid and you must pay for a new one on board. If you do that, tickets on the day of travel are the most expensive option always.