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Upvote for timeboxing your volunteering. If you can give your supervisor an idea of how many hours of work you can commit vs. how many hours of work you have on your plate, they'll be in a better position to line up more resources to take over some of your work.
+1 for "Try to make sure that they know how to find information they might need on your day off. You should have someone else available who knows where your stuff is and can look it up and answer questions."
"I don't feel like inquiring about the other position would give them a very good impression of me (and I really like this company!)" -- Why do you think so? In my experiences with internships, my supervisors wanted to help me find what I liked to do, and encouraged branching out.
