Kerrigan;
I have spent a little more time with the slave unit kit.
I think with some creativity you could actually drive two slave board kits from the lead A unit.
You could install one slave kit in the middle B unit exactly per the manual.
The front B unit would have the 10 wire pass-through cable installed, replacing the 4 wire cable there now. You could likely tap into this cable with all 10 wires of the inpuut cable to the slave board in this front B unit. This would effectively create a "Y" style pass-through cable.
Then theoretically any signals sent by the lead unit board would be acted upon by both trailing B units.
Since the power to drive smoke units and motors comes from the pickup rollers in each unit, there is no additional loads imposed on the lead unit boards; at least I don't think there would be.
Again, I would strongly suggest that you contact MTH for a definitive answer to this question.
Regards,
Rod