The squirt bottle is what works for me, but I do know that when we got Ernie, I had to be careful that my scolding Buford, didn't cause him to be jealous. I think some behaviors just have to be ignored when bringing a new one in. It stops with time.
I feel like I deprived myself of some joy because I made myself a nervous wreck. Just relax and enjoy. Someone told me they would work their issues out, and they have pretty much.