Map Multiple Objects and then Join into One Object with One UV Map? I am creating this brass table with glass shelving model for a game. To avoid breaking things, I can only have one mesh for the table frame and one for the glass shelves. I currently have a bunch of rounded rectangles for the frame and three flattened cubes for the shelves. The frame with legs is one object/mesh and the 3 shelves are another. I also need the whole table frame on one UV map layout and the glass on a separate one.

I tried joining all the rectangles for the frame and just doing Unwrap and it seemed to work but then all four legs were not mapped. So the only other solution I could think was to map each object separate and then join it all together after. Is there any way to do this where they'll be all on one layout? I was looking at the multi object uv edit add-on and it looks perfect but I really need to join everything in the end to what I described so not sure if that's going to screw it up.
Unless I am misunderstanding, you are just trying to preserve the UV mapping of each model while joining them to a single mesh?

If that is your goal, ctrl+J with both models selected in Object mode will do that.  The join will preserve the models UV maps as is.  It might get a bit weird if the two objects share the same material but the UVs will not have changed (though, they will be stacked on each other, but again, if the materials are assigned correctly it won't matter).