ELECTROMAGNETIC COUPLERS FOR MTH F3

One issue you'll have is the trailing A unit will be farther from the B unit.  If that's not an issue for you, it's simple.

The rear coupler is fed directly from the tether, so if you tap off  the two wires for the B unit coupler and add a switch to enable it only when the trailing A unit isn't connected, that will do the trick.  The coupler is connected between PV on tether pin 2 and the coupler output on tether pin 7.  You can get the wire colors from the rear connector on the B unit, on the replacement cable I have they're purple for PV and red for the coupler.

Since this is an ABA set it won't matter.  The simplest method is as John stated to open up the B unit and tap into the harness to get a PV and coupler wires.  Trace them to make sure you have the proper ones.  Take the control wire for the trail A coupler and via a switch connect it to the middle post.  The end post go to B unit coupler and Trail A coupler harness connection.  This is wired similar to how AUX steam tenders are wired.  Your selection via the switch controls which coupler fires.  G

That's an even better idea George, a SPDT switch eliminates the possibility of having the two couplers paralleled.