Kitty wrote: They are slowly adjusting to each other. Grommet REALLY wants to be friends with Carter, but Carter gets irritated with Grommet and just tries to run away from him. It's kind fo sad, really.
Back when I thought I was getting guinea pigs, I read that if the "old" guinea pig isn't reacting well to the "new" one, it might be because he's territorial. Apparently, if you clean out the cage (to get rid of urine territory-marking, etc) and rearrange all the toys/beds/dishes/whatever, and then place them both in the cage at the same time, the "old" guinea pig will think he's in a new cage and won't have territorial issues with the "new" guy.