I have created a simple workflow which loops every 5 minutes until two fields on the current item are no longer "empty" eg.
Loop while: value 1 is empty AND value 2 is empty
The loop ends once value 1 is populated and value 2 is still empty. I've tried a few different variants on this loop, and I get a variation of results none of which match the actual criteria.
workflow 1 status is not "completed" AND workflow 2 status is not "completed"
workflow 1 status equals "In progress" AND workflow 2 status equals "In progress"
I have even tried reversing the logic, however this just exits the loop immediately.