Hi,
Codalab doesn't work that way unfortunately. You can't have two simultaneous phases.
I know it's unnerving, but the practice phase shouldn't offer a great deal over and above the offline scorer.
If your submission to Codalab fails completely (for example you forget to compress the answers file) then it should allow another submission. If you have a good reason why you should get another submission you can ask us and we will consider all reasonable requests.
Genevieve