Auto Populate a Combo Box with Values from A Different Combo Box

I have two date fields, the first is a START_DATE and the second is an END_DATE.
 
Both are combo box (drop down). In the Combo Box Properties, under the Options tab, the Item List for both fields contain the same values (only Wednesdays are listed formatted as 2011/04/27).
 
When a START_DATE value is selected by the user, I want the END_DATE field to be updated to the same value. This is easily done using the guidance on this site using Thom Parker's tutorial named Changing Another Field with Combo Box (Drop Down) Selection.
 
The question is, can the END_DATE field later be changed to a different date selected from the drop down list of END_DATE.