Ah yes, this is a bit tricky. Look at sim/electrical/cross_tie_on, ties the two electrical buses together. If using something like ArdSimX, data.cfg for a switch on pin 53 would be like following (doing two functions on one switch flip)
D53- sim/systems/avionics_off
D53+ sim/systems/avionics_on
D53- sim/electrical/cross_tie_off
D53+ sim/electrical/cross_tie_on