It’s not ideal. And we realize your human won’t want to give either of you up. But they’ll need to put the work in to feed you with separate bowls, and regularly disinfect surfaces and litter trays. The chance of infection is low (so long as you don’t get in a fight) but that’s no excuse to take risks. You’ll also need enough space to get away from each other, to reduce stress and prevent scraps. And if they’re looking to adopt, they might think about picking a cat who already has FIV, to minimize the risk of spreading the disease further.