If you designate one end as the master, and the other as a slave, and the slave only responds when the master asks for data, your scenario is trivial.
Even if both are free to talk at the same time, the chances of collision are related to the amount of time each spends transmitting. Don't try to have one read War and Peace to the other, while the other is reading the Old English dictionary back.
If there IS a collision, the XBees will try to send the message(s) again.