So we have DCat, recently 3 years old, who we got from Cats Protection - she's wonderful, a perfect little cat who is the right balance of affectionate/aloof. We're in a first floor flat so don't have a secure garden, so we keep her indoors (mainly because she's a black cat, and there have been issues around us of black cats being stolen/abused 
rather than issues of her being able to get in and out, IYKWIM).
Would we be completely insane to consider getting a second rescue cat? The main reason for wanting another is the hope that we'd end up with another cat as lovely as the one we have.
We have space so they would be able to escape from each other/have their own space, but I keep thinking that cats get stressed out by having another cat in their home? (My mum had two cats and Cat1 was never the same again once Cat2 was introduced. MIL had two cats and Cat1 is much more affectionate since Cat2 passed away.