Do a husband and a wife need to have the same Rabbi (Posek) or can they have their own Rabbis?
Rabbi Hershel Schachter has an mp3 on yutorah.org on "Kitchen Kashrus" where he says that they could each follow their own rabbi on something like opening bottles on shabbos.
For matters of communal customs, e.g. how many hours to wait after meat before milk, or whether to eat kitniyos, they should belong to a single community; assuming he had a strong communal background, that's usually his. (Rabbi Schachter states that if the wife has a strong communal background and the husband is new to observant Judaism, he should join her community.)
I'll just add that practically, for "family purity" matters that affect both of them, they kind of need to figure out who they'll follow.