As you say, the problem is that shafts are too close in this class of subs. Of course in forward reducing speed in one shaft will greatly reduce turning radius, but starting from dead in water, making an "on position" turning may be quite difficult and slow. For Reducing turning radius I've tried "lexan" extension of rudders and have worked nicely (on 1/144 SEAWOLF and 1/96 AKULA II). I also have a 1/50 supply ship made from scratch, with twin rudders and shafts and bow thruster: Of course this boy not only turns "on a dime", also can be moved sidewards, cause propellers and rudders are considerably more separated than in a sub and "torque" is much bigger. Also consider that props and rudders are deeper in a ship than in a surfaced sub, and that also affects their effectivity in manoeuver. My proffesional experience (10 years in merchant marine, two of them as Captain) has taught me that turning over the axis starting from dead in water is almost impossible if you don't have, at least, double and well separated shafts and rudders (and calm weather, cause wind also affects evolutions).
Regards.