I would figure that the 180º phase difference between the legs would work in favor of tying them together. Perhaps parallel instead of serial...?
I wish I had paid more attention in physics...
Well it's not 2 phase power for a 220v device wired directly across L1 and L2, it sees a single phase voltage. My clothes dryer has just three connections on it's power cord, L1, L2, and a safety ground that carries no current unless there is fault/short condition. So it's just a simple 2 wire 220vac circuit for 220v devices and if you have two CTs measuring the same 2 wire circuit, how can adding their outputs together not give twice the true current flow value? Forget the 120 loads for the moment and just think about simple 220v loads.
A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. It is the AC equivalent of the original Edison 3-wire direct current system.
And to complicate it more, I bet you need to rectify and filter the CT's secondary voltage to actually be able to measure it with a standard micro based system, so all phase information if even useful is lost. Oh what evil webs we weave somtimes.