Then the results should be the same as you suggest, at least to my understanding.
I would make sure that the models are the same after 1 and 2 update steps just as a sanity check. Maybe Pytorch does something funky like a copy when you set two modules to the same, I’m not sure 
self.layer4_1 = model_resnet.layer4
self.layer4_2 = model_resnet.layer4