Sniper, you can't use transformers with DC current. Plus transformers in AC isolate the voltage potential, but the primary is still loaded by the secondary, plus the additional efficiency loss.
Putting four motors on a Heng Long board will result in one of these options:
1) Half voltage to each motor (series wiring) with resulting half speed...blech.
2) Double current to each motor (parallel wiring) with resulting smoking Heng Long board...double blech.


BIGMIG,
In the pic below, the orange wire supplies battery power from the driver board to the RX (small) board. If you want to run two separate batteries (1 on each driver board), only one of these wires should connect to the RX board (disconnect the other). Note that your wire may be a different color.

I'd actually highly recommend you either use one big battery or use a y connector and use your two batteries in parallel..this will prevent this problem and help ensure that the motors receive the same voltage and prevent any variation in load/current between the front and back pairs. Just connect the vcc (batt+) and gnd (batt-) between the boards.

Cheers,
D.