Yes you can do it. I think the only way to determine if there is a second anode is by removing it, and that seems out. The issue with mixing is that generally the idea of two is that you double the rust protection. Magnesium and aluminum are dissimilar metals, so the magnesium one will be used up more rapidly protecting the aluminum one. Unless you share our reservations about aluminum, you might consider replacing the main anode with another aluminum one. We don’t like them, and I don’t sell straight aluminum ones, but it seems an expedient thing to do in this case.