Here's a question: where should you source the power for your xfeed contactor coil, left batt coil, right batt coil?

Since at least one side of the Contactor would have BUSS Power (other wise there would be no need to actuate the contactor), you diode feed the + from the each side of the contactor to the coil. Then supply a ground to actuate. A fusable link in series with the diodes would provided adequate circuit protection.

[r.t.s. ]