EDIT: Got a winner! You can use instances to do this. Like attachments, naming is arbitrary but order is not
My previous "answer" was not great, but some may still find it applicable:
The closest thing I could think of to a solution (not for me since I'm using CartoDB and not MapBox) is to use font icons, which MapBox Studio now supports. You could bring in something like IcoMoon or FontAwesome if you have relatively standard icons, in which case you could use a marker as your background and the font icon as your foreground, both of which could be styled.
I couldn't tell from your "can't work out how to display one on top of the other" sentence if you already tried stacking markers, but if not then try something like this:
Great manual on CartoCSS here. Hope this helps, keep us posted if you find something!