Why would my garage have separate ceiling joists suspended below larger floor joists?
You ask, “Has anyone heard of this before?” Sure and it seems very logical to me. They are using the lumber where it’s needed most: the bigger joists to support the heavier snow loads and the smaller ceiling joists to support the lighter ceiling finish. Also, the deflection from the snow will not affect (crack) the ceiling finish because the structural systems operate independently of each other. 

Now, you want to change that. You’re going to tie the two structural systems together AND add a superimposed load. Hmmm...