I would do the butt on the bottom rack and the ribs on the top rack. The butt you won't have to fuss with until the ribs are just about done. Once the ribs are done you can just take the grate out and do what you need to do to it.
The fat dripping over whatever piece of meat really won't matter. Just go with what cooks the quickest to the easiest thing to get too and pull out, so you dont have to keep the smoker open for longer than you absolutely have too.
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