I was not able to reproduce this. Seems to work well for me.
But you could try to create iterators from the generators and use them:
for param in iter(model.parameters()):
    par1.append(param.grad.view(-1))

for param in iter(model.parameters()):
    par2.append(param.grad.view(-1))

If that doesn’t work you could look into copy and deepcopy to copy the parameters.