see the list here
https://www.sleeper.scot/tickets/railcard-discounts/
But basically i think they operate a system where their berth inclusive fares (which are probably their cheapest) do not permit '2 together' railcard discounts, but they do permit them if you sit up all night....(ie not use a berth)
So the option is then to search about for nights they sell cheaper all inclusive berth fares (I guess, book well ahead, avoid fridays and sundays - maybe look for tuesdays or weds when demand may be lower)
Not sure if the current operator does the 'Bargain Berths' cheap fares anymore that their predecessor did, but others will know if they do or not
OR Book a walk up ticket (off peak - the old Saver type ticket) and get the railcard discount on that, then pay the berth fee on top. This allows you to have a berth one way, come back on a day train the other way. But there is no railcard discount on the berth fee. You migth need to book the berth fee (which is flat price and not priced subject to demand AFAIK) in good time to be sure that all the berths do not get sold out, but just book the required rtn ticket a day or so before - so if things go wrong you do not have to get that refunded.
Use BR fares website to price that ticket from London or Brum etc as a bechmark.
Given the all inclusive price on the 1st option, and they can only sell the berth once per night so yuo don't save money getting on at Crewe AFAIK, if you can get to Euston cheaply to start your journey you may find that better than going to Crewe and getting on in the middle of the night, thus getting a better journey experience, more sleep etc by getting on the train earlier in the evening.
But I fear you will not beat a cheap flight price if you want a bed, however, it does save the cost of a B&B so you could maybe offset that against your ticket price?
ie Price you quote with berth = £200.
versus
Flight price 2x£50 plus circa £50 for that night in B&B / hotel = £150,
so is the sleeper 'experience' worth the extra £50 (or £25 each) to which I would say "
yes" but that's a personal judgment. You will of course be able to do your own calcs along these lines knowing your full costs.
Might help to read the 'how much does it cost' section here:
https://www.seat61.com/CaledonianSleepers.htm