Tbh, I think the 'nice room' scenario is just for the relatives' benefit too. Neither my mum or MIL were/are the slightest bit bothered about by who or what was in theirs. Same with an en-suite - as long as there is a washbasin, for hand washing, then it's not an essential either. Mum had her own toilet but ended up stuffing things down it and flooding the place, so I can see it's not without its problems when dementia residents are involved
In fact, my mum ended her days bed bound and it would have been nice to think someone else was around, rather than her lying there on her own. That thought was one of the more upsetting times for me and was the first thing I asked advice about on this forum.
I suppose my only concern would be if the other person wanted music or the TV on when my parent didn't. But if they're both very quiet, then I should imagine it would be OK. I doubt the CH would have suggested it unless they thought it would work. And most elderly people seem to like the company when they're in a shared hospital ward.
Only issue further down the line would be if anything happened to the other lady, then you wouldn't know who would be in with your mum, but you always have the option of her changing her room to a single then anyway.