If your players are likely to team up and attempt to screw the rest of the group, they'll do that whether or not their characters are married.I would probably watch how the dynamic works with the other players. There may be some out there who will resent these two characters having that sort of bond. They'll worry that the two will form a coalition and dictate too much of what the party does, how they divide treasure, and so on. And they'll act accordingly.
If your players are likely to team up and attempt to screw the rest of the group, they'll do that whether or not their characters are married.