Some places will try to rehome pairs that have bonded. Could be an option?
I used to lock mu catflap after about 9-10pm, whenever Patch has come back inside. These days, I very rarely lock the flap because she is a self made indoor cat, also I am confident that my nieghbourhood is quiet enough for if she wants to pop out for a quick wander or do a poop in the middle of the night, she could. BUT my catflap leads to gardens back to back, also I only started doing this 3 or 4 years ago knowing Patch never stays outside overnight. So it really depends on your cats.
In short, if you are not sure, lock them in at night.
My neighbours used to lock their cat out if they were not sure what to do as they didn't have acat flap, the amount of time that poor cat was waiting to go inside... for the record, it was the bully cat from upstair and I really didn't like him, but that doesn't mean I was less annoyed by his humans locking him out all day / all night.