I don't know if you mentioned this earlier, but has the other cat been tested for a urinary infection?

If it's not medical then both cats may benefit from Feliway. It mimicks cat hormones that the cat gives off when it rubs its head or cheeks against something, so the cat is in its 'happy place'.

A little goes a long way. It is not sprayed on the cat directly - either on a place where it spends a lot of time, or it comes in diffusers as well. I haven't used the latter, but others here have experience with it.

Ask your vet about it.

Keep us posted!