group within a group?

simply can't figure out how to create a bus track inside another bus.

If I have several vocal tracks, some lead, some backing I put them all in one vocal bus (or "group track"), but if I want to have another group track with just the backing vocals Cubase takes all those tracks out of my general vocal bus.

Any way to be albe to route a single track into two seperate group tracks or any other way to achieve what I'm describing?