If the 3rd channel is also linear proportional then you can use it. It would not work if its only 2 position. You can use the same channel for your front steering to control another servo for the rear steering. You will need a Y splitter servo cable. This will control both front and rear at the same time. You can set it up so the tires all point at the same direction, or both front opposite both rear.

Dont quite understand your 2nd question. Are you planning to run 2 motors?