Straight out of college 2 friends and I did a trip similar to the OP's suggestion. The gentleman who is now Mr. CDTraveler and I were very much a couple, but our friend's SO was unable to come, so the three of us ended up sharing rooms as well as rental car, etc. Since we were all fresh out of school on very tight budgets, everything was split 3 ways. To avoid constant figuring out who pays what, we agreed on a total trip buget, and pooled the cash. One person was "banker" and held the money and another was "treasurer" and kept track of expenses, receipts, etc. and actually paid the hotels, restaurants, and so on.

Must have worked out o.k. because we're all still friends many years later.