As I indicated above, it's not really clear enough to me what the requirements are and how one would implement it. So I can't point you to where one would implement it. Likely it'll affect a lot of code in coredevice/suservo.py, gateware/suservo, gateware/rtio/phy/servo.py. As I said, not an afternoon job.
One possible implementation may be to decouple the tasks and just mix two DDS channels from different Urukuls.