Matt,
It’s hard to teach techniques by email, so I’ll just outline a couple ideas. Some details will be left out to keep this message under 10 pages.
Instead of sending the first guy down with a bag, feed out the amount of rope you need from the bag at the top (this will happen automatically if you lower him). Leave bag one at the top. The last guy ties the end of the second rope to the first rope and connects bag one to the ropes. He then raps on both ropes, deploying the second from its bag as he goes. To retrieve, pull down on the second rope. The weight of bag one (and the small amount of rope it still contains) will actually make retrieval easier. This system allows you to use the exact amount of rope you need from each bag.
There is also a system that allows all but one person in a group to avoid a multi-pitch rappel and knot passing. Tie the two ropes together through the anchor with the knot on the down side. Rig a belay device (or munter) on the anchor just above the knot and tie it off. Tie the end of the rope to the rappeller’s harness. Have them rappel until they reach the end of the rope. Release the contingency anchor and lower them the rest of the way to the ground. Last guy down goes multi-pitch, passes a knot or uses Koen’s (scary) shunt release system.
Rich
P.S. These techniques will work in wet or dry canyons.