We plan to purchase the ScotRail Two Together pass for our September vacation. Is it advisable to do that on-line now or is it easy enough to do when we arrive in Edinburgh? Also, is it likely that some train travel in mid-September will necessitate prior reservations?
The Two Together Railcard is not a Scotrail discount but a National Rail railcard - valid on almost all railway companies throughout the UK.
You can get it in advance if you have UK details; it is just as simple to get in person over the counter at any National Rail station. If you provide the photos you hand them over with the form and £30 and they will make you the lovely purple card right then. Takes about 3 or 4 minutes if you get chatting, 2 or 3 if not,
You don't need the Railcard when you are buying tickets as long as you have it when you board the train. If your tickets are examined at a staffed barrier you should have it ready to show, if your tickets are examined on the train (could be either or both or neither) you need to show it to the Guard/Train Manager/Conductor/Ticket Inspector/Revenue Inspector when you show your tickets.
The Railcard gets you 1/3 off most tickets after 9:30 M-F or anytime on weekends and Bank Holidays. Restrictions at https://www.twotogether-railcard.co.uk/using-your-railcard/travel-times-tickets/ . Note that it is a discount against normal tickets, it is not a "pass" - there is no included or free travel with any Railcard.
As far as reservations are concerned, I know much more about English trains than Scottish ones so I expect that others will chip in - you don't need reservations on most trains. You will need them for the Sleeper to London and you may want them for long journeys (Scotland is a big place and some journeys take a long time) or when there is football or a special festival on. Many trains don't offer reservations, and by no means do all trains offer first class. When you are looking for tickets, we don't have first and second class the way the other European counties have, we have first and standard, if you wonder what standard means.