I keep reading that the best way to combine the two is to drape the quilt over the sleeping bag. This is especially true if it's a synthetic quilt. Your body moisture will end up in the quilt, and if it's synthetic it won't lose its loft as much as down will. I didn't bookmark anything and the only source I remember is on the Mountain Laurel Designs website (description of their synthetic quilts). I have seen this several other places, though. Also, as Jerry said, with the quilt on top you won't be compressing the insulation of either.
Depending on how cold it gets and whether you're a warm or cold sleeper, you will probably have to supplement the sleeping bag and quilt with warm clothing and/or perhaps a vapor barrier. Remember, too, that you'll definitely need a warmer sleeping pad or pad combination, with R value of at least 5.
You might want to try this out close to home or your car first, so you can bail out instead of shivering all night if it doesn't work.