(Louisville, KY, USA)
I have one very timid cat, Cleo, who hates when ANYTHING changes. I also have a big former-outdoor-cat named O'Malley who is very laid back.
Recently, we moved into a new house, and O'Malley seemed to want very much to go outside (the house has big windows and storm doors that he can look out). So my fiance and I thought we would try a harness and leash, since we live in a large city. He really enjoyed being outside.
The problem is not with O'Malley. When we came in, Cleo (who NEVER makes any kind of hissing or growling noises) totally freaked out, hissed and spit and basically did her impression of the Exorcist. All of this was directed at O'Malley, who is completely bumfuzzled and keeps trying to get closer to her. Which starts it all over again. We thought it was the harness, so we took that off as quickly as possible.
It's now the next morning and she's still freaking out every time he gets near her. Is it just his new outdoorsy smell? Or is Cleo just having another "I don't like change" issue?
Both. Change is very stressful to a cat. And yes, when you come back indoors, O'Malley smells very different and perhaps she even sees him outdoors and perceives him as being another cat.
So, the answer to your question is simple. Yes and Yes. The solution to the problem is not so simple.
Just waiting for time to pass will help probably more than anything. Extra attention to Cleo would be helpful. You may also try adding more toys, some catnip, and Feliway plug-in's and spray.
Also, if you take a towel or article of dirty clothing of yours and rub first one cat down then the other then back to the first and then the other, you will transfer scents back and forth and that can help enormously.
A tremendous help would be simply not taking O'Malley outside ever, but I don't know if you are willing to consider that.