A berth on the Caledonian sleeper is £75 each and this entitles you to a berth in the same class as your ALR. For example, if you have a STD class ALR and you book a berth supplement for 1 person you will get a STD berth in which you could end up sharing with someone of the same gender. Of course if there's two of you then you'll just pick the same berth and use the 2 beds so you'd be together. If you have a 1st ALR then you'll each be assigned your own berth. If you have a 1st ALR and wanted to share a room then i guess you could call them / email them although they're not always the fastest to respond to emails.
As for the night riviera, i think twin berth pricing structure is a little different. You'd have to check the GWR website for this i think. I know an upgrade with an ALR is £60 each, these are for single berths.
Rob