Am I Understanding How to Buy this Train Ticket Correctly?
 

Will be buying a one-way first class ticket from London's King Cross to Edinburgh for 23 May 2017 for 2 people. Right now plan on taking the 11:30 a.m. train. It looks like the Two Together Rail Card would give us a nice savings.

I see from the description of the railcard that because we live in the U.S. we will need to get the card at Kings Cross because they don't mail to the U.S. I found the information on how to download the application and will get photos.

Am I understanding correctly though that I can go ahead and buy the tickets online in advance indicating I have the railcard and then simply ensure we do get the railcard so when tickets are checked we present both?

Yes, exactly. Note that certain tickets aren't eligible with Two Together card, esp some peak-time trains on weekdays (usually those leaving before 0930).

For the best scenery which begins after Newcastle try to nab seats on the east side of the train - can see a bit better with a window right by you.

I've done exactly that twice. Be sure to leave enough time at the station to buy the two-together card, there is often a queue and it must be done at a staffed window. Or, you can buy it at any station in London before the day.

Great. Thanks all

Be careful about the free-flowing booze gratis in first class -some have reported being a little tipsy before Edinburgh! Complimentary snacks too.