Without a bunch of testing, I would say you could create a second set of combo boxes to work in reverse order or create a second form with the boxes working in reverse order.
If you do two sets of boxes on the same form, you could put a box around the combo boxes with directions on how to use them to search or put an arrow leading from one to the other.
The reason for using one of the above options is the query feeding the second combo box is based on the first box having a value (see post #6, second comment 5.)