THIS (Click here)
is what's holding them together. You'll need to pull both units out together so you can get to the back of the machines. It's the same concept as stackable units used in a house. The clip attaches to the bottom and then the top has a groove that slides into the clip. The front is then snapped in place.
On that web site page, look at the "Installation PDF - Instructions" link for a a diagram of how they are attached.
Who you are right now is a sum total off all you use to be.
2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually