In layman's terms, it looks like:
1. The motor and headlight in each engine get connected to the power wire from the intake rollers and to a body ground;
2. The 2 wires between the engines connect the respective brush points?
So there's no direct connection from the remaining E unit to the other engine's motor?
I'm not clear on #2.
If you look closely at the second unit, its dual field connections are reversed from the first units motor, ie the "upper" coil is connected to the seconds "lower" coil and vice versa, so that second motor runs in reverse rotation relative to the controlling unit motor rotation, insuring first and second units move together. Basically wiring both motor fields in parallel except second motor field coils are reversed wired. (sorry so wordy)
I would use a small polarized connector on tether so you don't have to carry both units around, or if you want to run controlling unit by itself. Insure the female socket end is on controlling unit, as you would not want male prongs touching ground or center rails.