Bernhard, there are a couple of ways of doing what you want. The simplest is to use what’s known as “Tank steering”, where each motor is controlled by the vertical movement of each transmitter stick. I use Mode 1, which means that the throttle control is on the right stick, so I use that to control the right hand motor. I then use the left stick (which, if used in an aircraft would control the elevator) to operate the left hand motor. If your radio is Mode 2, then the throttle would be on the left stick, but the same principle applies, that is, right stick = right motor, and left stick = left motor.
The other alternative is to use some sort of a mixer. In my patrol boat, I opted for an ACTion Electronics P94, which has 2 speed controllers and a mixing function built in. It also has the capability to run a bow thruster. Have a look at the ACTion web site mixer page here
http://www.action-electronics.co.uk/mixers.php It is a marvellous piece of gear, not much dearer than 2 ESCs, and I highly recommend it. A photo of my installation is below. The P94 unit is in the black box, the other bits are also from ACTion, including a fuse block and a power distribution board. The Mtroniks ESC is for the bow thruster.
Dave Milbourn, the proprietor of ACTion, is a member of this forum and is a very knowledgeable and helpful man, who is more than willing to give advice, if asked.
Peter.