Thanks for the nice and detailed explanation. I prefer to do two  Dataset classes , and I do like the answer (and the idea) you are suggesting. Hence, the testing should be something like:
for batch_idx, features in enumerate(test_loader_1):
       # evaluate model on test dataset #1
       # test_loss += criterion(...)
for batch_idx, features in enumerate(test_loader_2):
       # evaluate model on test dataset #2
       # test_loss += criterion(...)