One tab on one side is adequate to trim the elevator, and the forces are light enough that the imbalance does not create a structural problem for the elevator.
As you suspected it would impart a very slight roll moment on the airplane as a whole, but on a light airplane it's a small force and not really noticeable. Ideally an airplane should be rigged to fly straight-and-true at normal cruise speed, so the trim-tab roll moment might be cancelled out by the design and rigging of the other control surfaces.
Larger airplanes would probably not have asymetric control tabs.