given that the Sul'dam don't seem to use the a'dam the same way Nynaeve did. Nynaeve seemed to just directly access Moghedien's power and use it herself. Sul'dam seem to act like animal trainers and give damane instruction and have the damane do the actual weaving. if a Sul'dam was shielded I'm not sure either she or the damane would be functionally disabled unless the Sul'dam needed to punish the damane.