Similar to coupling across three pahse
applications, you can try to use phase
couplers to bridge phase one of load center
one, over to phase one of load center two,
and so-on for phase two of LC one and phase
two of LC two. Under these conditions you
will probably need a amplifier/repeater in
one load center (which ever has most X10).
I don't have a clue if this might be against
some electrical code, you will have to
research that.