If you want to use the mixer, you will require two independent speed controllers to run the two motors independently. I had the Caldercraft Resolve, which uses this arrangement. I set up a couple of variations of control while I owned it.
Firstly, I had the two motors, mixed on the LH stick of a four channel system, with the rudder, independent on the RH sticks side to side action. This was the most maneuverable option, but I did need to pay attention when driving the model, avoiding getting all fingers and thumbs! Usually I would operate the LH throttle up and down ( both motors operating together ), with the rudder control for steering in big areas. The turning circle was that of a pig! In close and on small waters, I would use the Left/Right function on the LH stick to mix the motors, using the rudder to aid the models tugs.
My other set up did away with the mixer and used both Up/Down functions on the set to operate the two motors, tank fashion, with rudder control on the RH stick as normal. I did find that it was easier to cruise about on one motor, adjusting the rudder to compensate, rather than keep constantly using both sticks and therefore both motors at all times. Good for making big bow waves though!!