I balanced my dual Holly Webers by using a Dwyer magnehelic gauge and some rubber tubing. Set up the gauge and then inserted the tubing (with a pitot tube attached) in the throat of each carb. Then just adjusted each carb to that the DP pressure readings were the same. Also need to make sure that the dual carb linkage is dead on and open each carb equally and at the same moment.

Country Roads, West (By God) Virginia
http://community.webshots.com/user/geezer