Um, Darren, I think that's was just what xyzer said. The only issue is that since you have two flywheels, and each (probably) has a different balance, you have to balance them separately, or you end up with a rocking couple between the flywheels, an over-compensated crankpin mass, or both.
I'm thinking a good place to start might be figure out the compensation mass, divide by two, then do the lever arm math for the rim of the flywheel from the crankpin. Then add that amount of mass to the rim of one flywheel in the right place and statically balance it. do the same with the other. Then bolt them up.