you can use apex_item api for this. create a region and have different calls to apex_item api based on your desired condition.
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.
You COULD have your LOV based upon a function returning SQL, thus the select would be built of code that you could make it more dynamic..
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...
Then you are looking for something BEYOND an LOV or even a dynamic LOV, you almost want the ability to have an LOV AND also ad new items to your list, correct?
Some what like that only but lov's to be enabled dynamically as per the requirement ...
You can create a dynamic action of 'show' for 2nd item (uncheck the fire load for the action).
You will have some condition for the 2nd item to get displayed na, put that as your condition under "WHEN" tab in dynamic action.