Looking, I see the 2 Together railcard is valid on the Caledonian Sleeper (seated accomodation only)...
Southern offer it for Thameslink trains Gatwick to Bedord running through 0430...
My assumption has always been that such temporal non-validity (in this case between 0430 & 0930) was always based on the (scheduled) start time of your journey - and often assumed that there would be no services running at the start of the non-validity period - which was often in the early hours when 'yesterday's trains' had finished, and today's hadn't started.
Methinks at times the railway's wording on such restrictions can be rather loose.
If on the other hand, the non-validity period is strict not valid for travel in this period, then there is always the risk of tickets becoming non valid if train happened to run late...
And if it is, a random question - what is the earliest train on which a 2 Together ticket would not be valid on the grounds that arrival will not be until after 0430?