The last two lines of code would recreate the iterator, so you should split the calls to:
data_iter1 = iter(trainloader)
data_iter2 = iter(trainloaderNeg)

images1, targets1 = next(data_iter1)
images2, targets2 = next(data_iter2)
...

Note that you would have to catch the StopIteration once the iterators are empty.