One Return or Two for Interconnects?

Actually, it is the return wire that has some theoretical advantage to bi-wiring. (One signal and two returns).

If the return wire has any resistance the Low terminal of the woofer will have an attenuated (and probably distorted) woofer voltage on it, and, with a single return wire, this voltage is applied to the tweeter Low terminal instead of zero volts.

I leave it up to others to figure out if the effect is significant.

Whoops!!!... I see this is about Interconnects, not speaker wires. However, a similar argument would apply. There would theoretically be some loss of separation, but this would be miniscule because the input impedance is so much higher than the interconnect wire resistance.