Conventional helicopters have a tail rotor or equivalent to counteract rotor torque. The pilot's rudder pedals control the force generated by the tail rotor (e.g. by changing its pitch).
But the V-22 Osprey lacks a tail rotor: it has rudders on its twin vertical stabilisers. Rudders wouldn't seem to have any useful effect when in vertical flight, because there's no horizontal airflow for them to deflect. The AW609 appears to lack even a rudder, in the photographs I've seen: the vertical stabiliser appears to be one piece in a fixed shape.
Co-axial helicopters such as the Ka-50 can yaw by varying the torque of one rotor compared to the other, but it seems unlikely to me that the same mechanism would work well when the two rotors are at opposite ends of the wings. The effect of differential torque would be much smaller, and it would have huge secondary effects (such as adverse roll). For that reason I don't think the Osprey can work that way.
So what mechanism controls the yaw on a tiltrotor?