Kirt,
Both Bfly and Jorge have provided good information, so I will just recommend some modern research reading for you:
1. Read the Cal Pappas book, The Double Rifle Primer.
2. Look up on the Internet, the Ellis Brown Custom Shop. Buy and read the W. Ellis Brown book, Building Double Rifles on Shotgun Actions.

I know both of these gentleman shooters and they are quite knowledgeable hunters and shooters and know whereof they speak about double rifles.

Enjoy!

CRS, NRA Benefactor Life Member, Whittington Center, TSRA, DWWC, DRSS
Android Reloading Ballistics App at http://www.xplat.net/