Since "super+u" doesn't work, you can write a plugin for it.
def run(self, edit):
cursors = list(self.view.sel())
if len(cursors) > 1:
Didn't really test it, but it should work. It's pretty straight forward. I recall add_all not working in ST2, but that could be wrong. If it doesn't work, simply loop over the cursors and call self.view.sel().add(cursor)