Twin electric motors can be operated by various means. You can run two motors in parallel from one ESC or you can operate two separate ESC's from one receiver channel. In both of these cases the motors will run at the same speed and in the same direction together. If you wish to do what the full-size boats do however, you can split the two motors between two separate channels and use a transmitter stick to control each one independently. This is called 'tank steering' as it’s the way a tank driver controls his vehicle. It allows much better low-speed control for manoeuvring into a dock, and it can also greatly improve the turning ability of a model. However you’ll have spotted that it required two channels for the throttles and it’s also quite a complicated skill to learn.
You can alternatively fit an on-board device called a motor mixer, which is coupled to the throttle and steering outputs of the receiver and automatically slows down or reverses the inner motor in a turn as you apply a steering command. There are several different types on the market (e.g. Mtroniks, ACTion, Hunter) and also a combined twin speed controller and mixer, the ACTion P94. I’d suggest you have a look at the article: http://www.action-electronics.co.uk/pdfs/Twins%20Ver2.pdf, which explains and illustrates further how these units work.