Ive just checked my 30Amp electronizes and the negative supply from the battery to the speed controller is connected to the black wire to the receiver.

Assuming your 15A ESC and 30A ESC in your wiring diagram are all electronizes, I think you might have a problem.

Possible solutions (off the top of my head, might not be practical):
1. Use an optoisolator between the receiver and the ESC.
2. Modify the ESC pcb to remove the connection
3. control the ESC using a mechanical connection (servo driving a pot or similar)
4. use two receivers (electricaly isolated)
5. use relays instead of ESCs
6. use ESCs that are already isolated.

Simplest solution is probably to get an optoisolator. The only ready made solution for RC I can find is the zero glitch module (£8.99) sold here: http://www.goldstarstockists.com/tamiyaparts/tpx/electronicgadgets.htmhttp://www.goldstarstockists.com/tam...nicgadgets.htm
I wouldnt have thought its too hard to build your own using a cheap IC though. Somebody else whos had a go at this could probably chip in with some advice here.