Years ago when I asked about this same thing,(2 powered 2343's, 4 motors ) I was told to 'lock' one e-unit in forward, and the other in rev. makes sense but ya can't back up.
This can be accomplished easily by locking the slave F3 in neutral, and a three wire tether(brush-field-brush) from one slave motor to one motor in the master F3. Switch the tether brush connections if they're going in opposite directions. All four motors will run on the master E-Unit.
Lionel has maintained over the years that the 259E-1 architecture will support 4 motors using 22 ga motor wiring.