Interesting question. I guess whether it is expected would depend on the scene. If it was toward the end of the session and this was the finale, it may be expected that the dom would instruct her slave to masturbate. I've made videos in which a slave is whipped by one mistress as he worships another one. Whipping and body worship required two doms. I can see how it cbt could could be simultaneous but...whipping on the back or the ass...how do you picture it working with one dom?