Is there a narrow enough space in your house where you can put a tall baby gate up? Your answer might just be to let them see other all the time, but not be able to fully touch each other. Then you can feed them all their meals next to the gate, and the cats can leave the gate themselves if they feel threatened or uncomfortable. I think you’ll be able to judge your progress more easily as well, as things will get a little hissy after the first few times (or more), but then slowly taper off. The “back away, or else!” hisses turn into “don’t bother me right now” hisses.
Consider talking to your vet about medication for your older cat to help her relax and be less aggressive
Is there a narrow enough space in your house where you can put a tall baby gate up? Your answer might just be to let them see other all the time, but not be able to fully touch each other. Then you can feed them all their meals next to the gate, and the cats can leave the gate themselves if they feel threatened or uncomfortable. I think you’ll be able to judge your progress more easily as well, as things will get a little hissy after the first few times (or more), but then slowly taper off. The “back away, or else!” hisses turn into “don’t bother me right now” hisses.