Sarah:
Sounds to me that you need to build a custom control or at the minimum three emems on the same ME bank.
Emem 1 being Box behind other. Emem 2 - bottom box moved to position #2. Emem 3 - Keyer priority change for your boxes.
Personally I would build it using a custom control.
Either way will give you the same results. I hope that helps
#Carbonite