Jeff,

The principle is the same: find the center of pressure where the lift of all the surfaces is considered applied, then select a stability margin and locate a safe CG.

This article may help you:

http://www.geistware.com/rcmodeling/...calc/index.htm

Now, the incidence angle of each surface is another matter and is related to longitudinal dihedral.

Andy Lennon;s book has a good description of designing a tandem plane.