I would always go for dual! As a single sub will have room modes and the bass will not be consistent between the seats.
Example here (Green and Red locations individually have awful dips in their response, but combined they tackle each others weaknesses and help to get a good flat responce):
This is both subs combined (before proper EQ):
Going will dual would help to solve those issues. BTW what receiver are you using in your setup? This makes big difference since you want one that can EQ each sub individually instead of just having a simple Y splitter built it.