Do yourself a big favor and keep the planes servos separate! Very few people have been able to "gang" them together and get a well performing boat. As Art stated, the stern planes should be for angle control, and the bow, or sail planes, for depth control. The cool things about model subs is that the bow planes can either function or be fixed on "0" degrees and the stern planes will handle the control of angle and depth very well by themselves. On 1:1 scale boats the bow planes are typically rigged in (retracted) for high speed operations, as they are not needed for ship control.
Take care, have fun!!
"If you ignore the problem long enough, it will go away. Even flooding stops eventually!"