The MiG 21 had all-moving tailplanes or stabilators. When rolling, did one stabilator pitch up and the other down to act as secondary ailerons and help the main ones on the wing?
MiG-21 (any modification) didn't need it and didn't have it. With its low-aspect-ratio delta wing and most mass concentrated in the centre, it was agile enough with standard ailerons.
The elevator had a single hydraulic booster driving both halves of the stabiliser.
Differential stabiliser was used on MiG-21's successor, MiG-23. That one had no ailerons at all; it had full-span flaps on its swing wing instead. Roll was controlled by the spoilers and the differential stabiliser.