Over the last 18 months, I have bought a zenoah kill switch and a mixer for twin motors from there. They work first time, are relatively inexpensive and come with good clear instructions.
With the mixer, I had a test rig set up for a proposed triple motor fast electric catamaran, a Graupner PX-1. With the gismo from Mr RC world and a couple of y leads effectively I had rudder and throttle working normally on the centre motor ESC and the rudder. The other two esc's worked fine , giving differential power as rudder was applied either way on the outboard motors, which is what I think Metal Mangler needs.
I also use the Futaba 6EX tx's and while it might be possible to achieve the same through channel mixing, I wouldn't like to try it, life's too short!