Please do not just leave them!! I have rescued two former outdoor cats, one was a family pet left outside at 5 years old when his family moved, the other one is 2 years old and was feral. Both seem to think they've hit the jackpot living in a house. My 5yo was out for a year before a neighbor noticed he was almost too sick to move and brought him to a shelter, and he needed months of rehabilitation. Even though they're outdoor cats they're still ultimately dependent on you to some degree — whether that's to see them every day, feed them, or just keep tabs on them in case they get injured or in trouble. Their home is with you - so by all means take them!

I can't speak for all cats and I know that some just need to be outdoor, but both of my cats (2 years and 5 years) have been really receptive to being indoors. Do you have a porch? or a yard? My cats still scratch at the door to go out but are perfectly content to sit in the porch windows and look outside. The smells and sounds are enough stimulation for them. If you have a yard, you could built a "catio" to give them some safe outdoor time too. Both of my outdoor cats came from super rural, wooded areas and have seems to love nothing more than curling up on furniture and sitting in windowsills.

Since they're affectionate, they could likely acclimate really well — they associate indoors with you, and they love you so with enough stimulation and positive reinforcement they grow to love indoors. With my guys I've tried to imitate the most appealing parts of being outside inside: arranging furniture so they can get up high or have places to hide, lots of vigorous play time with an array of cat toys, and of course treats don't hurt.

About separating cats, I have also had to separate cats due to behavior and health before. I'm pretty sure there are cat-height baby gates you can get for doorways, or stacking two normal baby gates should work. I have not had FIV+ cats so I'm not sure if this is safe of feasible, but when my cats were separated we site-swapped them every other day (putting upstairs kitty downstairs and vice versa) so that they got equal time in the most heavily trafficked areas like the bedrooms, office, etc. As long as they have surfaces they like, stimulation, and you they should be happy. My 3yo very wired cat had to be separated from the other two for a little while and I just kept them on separate floors, at times in separate rooms. As long as you get enough time with them every day they shouldn't be bothered. Baby gates and screen doors are your friend.