if one of you are going to play online then you can use the subscription account and get achievments, the other can play offline on a different account and get the achievments.
another option is to have different saved games, both of you can resume from your own saved points in a game while using the same profile but the achievments can still be only unlocked once on the same profile.
the only problem i see with having the same online profile is friends, your misses could be playing and getting requests to join games from your friends and vice versa, no big deal really.