Originally Posted by Larke
She's not 'going' because she's afraid the new cat will smell her stuff and find her (and attack, steal her food, whatever). And because she's suddenly confined in her own home. I've found that confining any cats except small kittens (in a 'new cat' intro. situation) is a waste of time (just prolongs everything) and upsets them all. Let them work it out in person once you've rubbed them both down with a towel and exchanged the towels. Make sure they both know they have their own boxes and dishes (pref. where they can't see each other eating) and if they hiss and growl and swat each other, they will. If one cat seems a lot larger, meaner and has a great advantage over the other one, then you maybe want to do a slow intro through a screen of some kind, but I've never bothered with anything (except with kittens) and have intro'd LOTS of cats, and no one has ever been killed or anything like it. The worst scenario though is when the 'new' cats (how ever long you've had them) decide to take over from the last ones you acquired (how ever many yrs ago) and that can get vicious, and can last for ages.
Part of the reason to seperate new additions from the established cats in a household is to provide a quarantine time. The new cat could be harboring and incubating an illness that isn't apparant to the new owners yet. A week or so gives a pretty good buffer and can go a long way to confirming that the new cat is healthy and won't make the established cats sick.
I think it's too risky to put cats nose to nose before at least a week has gone by to make sure the new cat isn't carrying something. The stress created by new additions can allow viruses to take hold much more easily.
The inconvenience of seperating the cats is worth the reduced risk of making your established cats sick, IMO.