The unit you posted the photo of is basically a control unit that mixes rudder and throttle input.... It has two leads that go to the receiver, one goes to the throttle channel, and the other goes to the rudder channel.
Pushing the throttle stick forwards will give you forward motion, pulling it back will go into reverse. Then if you push the rudder stick to the right, the right hand motor will slow as you add more and more rudder, until it eventually stops and then goes into reverse, allowing you to turn on the spot if you wish.
I assume that the unit has built in speed controllers? If not you will require two, one for each motor...
I'm not sure that my explanation really helps you, so ask again if anything is unclear and I'll try to elaborate!
Regards
Eddy