I'm not sure how well it works but you could try feliway spray.
Cat's also HATE citrus...but if they are using multiple spots it's very difficult to spray them all down with orange or lemmon.
Also are both cats fixed? If not that could be a HUGE source of the problem.
Here's an article from the veterinary online library for you to check out too. http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=633&EVetID=3001640