How does a tiltrotor yaw while in vertical flight? 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?
The V22 Osprey yaws by tilting one rotor backwards from the vertical and one forward by moving the swashplate up on one side and down on the other causing a cyclic change to the rotor blade angles of attack.  This is accomplished using the pedals in just the same way as a helicopter.

Since the rotors are tilted from the vertical, this introduces a horizontal component to the thrust vector.  The rearwards titled rotor produces a horizontal thrust component to the rear and the forwards tilted rotor produces a horizontal thrust component to the front.  This produces a turning moment, using the wings as the lever, around the centre of gravity and therefore the aircraft yaws.

The secondary effects will be as expected.  Since some of the thrust vector is no longer vertical, a little more power will be required to maintain height and I expect that there will be a pitch change although I'm not familiar enough with the aerodynamics of this type to figure out which way and how big.