In most cases, "the tail wagging the dog" is not a good thing. But in the realm of vessels that travel on or under the water, it is the tail that determines the direction that vessel will go in. Think about it. After thousands of years of trial and error, it's been determined that the best way to turn a boat is by moving the stern sideways to point the bow in the direction you want the boat to go. Hence, the rudder is in the stern. If you want your boat to turn better WHILE UNDER WAY, put the thruster in the stern and control it with a speed control connected via a "Y" connector to the rudder channel as Art Broder described.
As far as individual control of the screws is concerned, in most submarines (Typhoon is probably an exception), the screws are too close together relative to the length of the boat to really give effective leverage for turning purposes. I tried that on a relatively short sub a bunch of years ago. Ahead on the port screw/astern on the starboard screw. Put the transmitter down. Head for the head. Talk to a couple of guys along the way. Take care of business. Talk to a couple other guys on the way back. Stop for a sandwich. In that amount of time, the boat had not yet reached 90 degrees! As soon as I got home, I pulled the extra speed control out and built an operating periscope system controlled by that now extra channel. Much better use of a channel!
The U.S. of A - Land of the Free BECAUSE of the Brave