Without knowing more about your workflow and reason....
You could have both fields on your form, and display one or the other depending on your condition, with one set to default.
Using the drag-and-drop view set both side by side. Set both items to have the same width so it doesn't affect your layout.
Both item fields would populate the same application item and use the application item in your workflow.
I have 5 items with different lov's for 2 items...
for employee number
nd for department
If i select the employee no then automatically loads the remaining values to the respective items(for this i had a dynamic action)
but if i select department then that employee no should be free text...