I thought I remember that the crown would sum. That's probably not mono mode, maybe there is another mode where it sums?
Having said that, for this application, I think it might be fine just to feed either left or right by itself to the tranducers. They should shake your ass just fine that way.

EDIT: Just checked, I was wrong, no way to bridge and combine. You'd have to feed one channel only to the crown. Unless you put a "left" connected transducer in one seat, and a "right" in the other seat.