Hmm, at first I was going to say there's no simple way....
I suppose you could place two copies of the field on top of each other on your layout. Give them different object names and remove the bottom field from the tab order.
Set an OnObjectEnter script to fire when the top copy of the field is entered. In the script use Get (ActiveModifierKeys) to detect the key. If the specified modifier key is down, use go to Object to place the cursor in the bottom copy of the field.
Now just assign your two different value lists to each of the two copies of your field.
I was stacking those fields when I received your response!
Thanks Phil, you confirmed my hunch that this was the most simple solution.