Yes for the helper engine if behind the lead engine. The lead engine will control the speed of the the train regardless if it is the assigned train engine or the helper assignment. But since the two engineer are so near seeing and hearing the power of the other engine, they will work in sinc with each other.
Great example of all this was the Western Maryland north of Elkins, WV where 10 2-8-0 were on one 100 car coal train. Three on the lead, 4 in mid train and 3 on the rear. 7 were in full throttle while the 3 on the point controled the speed with the power they added, all lead by the lead engineer viewing the track ahead, and also controled by his whistle signals, and air brake valve on the 3.04% grade.