Hi Shawn,
First off, I apologize for taking this long to answer your post, I had to think about your options for this one a bit. Unfortunately, I don't see any way to do this using the stock tables/fields. If this is something you really want, then you will have to Customize the "CustomerEquipment" screen, as well as the CustomerEquipment table. For what you are trying to do, it sounds like you would be able to add some additional fields to your CustomerEquipment Table, like "secondaryEquipment". Then, you would have to customize your CustomerEquipment screen accordingly to add these additional fields, as well as add whatever logic you need to pull up secondaryEquipment after a main piece of Equipment has been selected.
I hope this helps you visualize what would be needed to get this working. If you have some more specific questions as you attempt this, please don't hesitate to post here.
-Ben