Add a DPDT toggle switch to the Y-axis signal input (between the ILDA connector and the scanner amp) on one of your projectors.
Wire it so that in the "inverted" position, the switch swaps the signal leads to the amp.
(or, swap the leads at the ILDA "out" connector itself, which will then affect every projector plugged into that connector)
Several of us have done that using the X-axis inputs, and it works great!
RR
Metrologic HeNe 3.3mw Modulated laser, 2 Radio Shack motors, and a broken mirror.
1979.
Sweet.....