Here's a thread... http://www.bikeforums.net/showthread.php?t=121690

In your specific case, you'll need 1.5 roof racks if you want to be able to mount the bikes on the roofs of both vehicles. The bike mounts and the cross bars can be moved, but the clips are specific to the car.

If you guys can agree to use one car for all your bike transportation needs, definitely go for the roof rack.