Believe that this activates via steam.
So long as you're both on your own steam accounts, it shouldn't matter what IP you guys are paying on (i think).
I own it already and am buying it for my girlfriend so we can play co-op so if it doesn't work that way I'll be pretty irritated. and surprised.
Yeah, me and my wife both play online co-op together and there's no issues. You want to select local co-op (probably much less lag) and you generally have to be in the same area and statis level to join each other.
One thing to note, you have to buy this through two seperate GMG accounts - they only allow 1 purchase per title per account. Rather dumb policy.