The GFCI breaker will see the imbalance, and trip. Remember that the GFCI is looking at the difference between the current on the hot and the neutral on the load side of the GFCI.
I don't see an imbalance here. Example, heating cable #1 draws 7 amps, heating cable #2 draws 10 amps. Neutral would carry the difference of the two, which is 3 amps. As Steve says, the CT will enclose all 3 wires and read "0" unless there's leakage to ground. The example you gave has the dishwasher on a separate circuit (not on GFCI). Because the neutral was shared between the GFCI protected circuit and the other circuit, there would be an imbalance - the GFCI breaker wouldn't "see" the current in the hot wire for the dishwasher, but would "see" the neutral current.
No offense Techie, but I'm siding with Steve on this one - at least until someone can convince me otherwise. I'm quite sure that this will work using 3-wire and a double pole GFCI breaker and splitting the hot on each receptacle, but I want to be 100% sure. It makes for a good argument.
