I guess one thing to consider is WHY do you want to add another grey? Is it because your wife wants a bird that likes her better or some other reason.
I have a quaker (he was first), a gray, and an amazon. Another thing to keep in mind is their relationships do not stay the same. My quaker has recently decided that Brutus, the grey, is boring, so he is fixated on poor Pancho. Jimpster, the quaker, is a policeman, he likes to control EVERYTHING! Brutus figured out how to deal with Jimpster, so he has moved on to fresh prospects. Maybe it is because Jimpster is now 5 years old and in the prime of his years, but he certainly is a noisy force to be reckoned with, and only just lately. geez.
If i were considering another bird and I only had a grey, I would definitely get a DIFFERENT bird than a grey. Brutus is smarter, more clever than the rest, and he does want to be worshipped. I cannot imagine TWO like that! That would totally keep you on your toes!