We split evenly unless there's a big discrepancy (i.e. one couple drink a bottle of wine and the other only has water), then we do it separately. Although it'd be more like:
total was $100, wine was $20 so one pays $40, the other $60. Meh. We round everything up to make it easier though. We're pretty easy going though and more often than not, someone just offers to cover the whole thing, and next time we do the same!