Parasitic drag will surely be larger for the T-foil rudders, but the end-vortex of a single foil will be reduced by the T-foil. Winglets on airplanes give a small drag reduction, so small that some modern designs dont even bother with them. In addition the angle and shape of the winglets are critical to make them perform, and this is on airplanes which operate in far more stable conditions than I think a rudder do.
I dont have the math to calculate the differences in drag, but dont believe the drag is the key to this. I believe it is the increased effectiveness of the rig.

Here is a very good article on the development of winglets.
http://oea.larc.nasa.gov/PAIS/Concept2Reality/winglets.html