Quoted from SilverUnicorn:
Thanks for the well wishes! Hobbes is NOT getting along with his brother AT ALL. They really got into it tonight. It's like they never knew each other. More stresses. Keeping them separated and trying to do scent swaps to get them acclimated again.
I've had that happen before. I've heard that sometimes when the one cat that came home from the vet may not smell the same to the other kitty (but you probably won't notice) because of some of the medications it has either ingested or been injected with. Weather or not this is true, things get back to normal in a day or so.