Examples of Dual Mount below:
This Mount was designed to be able to change the position of the Stack Top or Triple top to a Rearward or Forward position to better balance the units out evenly. For the Bigfoot II, there is only one height: 4 3/4" Tall. Having the ability to change positions can come in handy to help the closest unit not hit your shin whenever you can't move the mount forward due to the Bow design of your Boat.
Looking for a Dual Stack? Then go to the Bigfoot II - (Adjustable Stack / Dual Stack Mount)