Are they both spayed? Don't know if it makes any difference.

Also - I heard on here about a vet who boarded cats and let them out together...he dabbed just a bit of aftershave/cologne on each forehead so they all smelled the same.

Might be worth a try.

Also, call your vet and get their take on things. There are some good tips archived on CatAge.com as well.

Catty1