hi!
sorry for the late reply.
i haven't seen this post until just now.
you' ve probably got this by now, but if not and/ or if someone else wants to know...
in the mixer (shortcut is F3), select all tracks you want routed to the group (bus), using shift or ctrl + click.
when you have selected all tracks you want, right click and choose "add group track to selected channels".
a window pops up with some options.
one of the options is how many group tracks you wanna add.
if you choose to add 2 group tracks, you'll get what you want.
if you wanna add more tracks to the groups later on, you can do this in the routing section in the mixer.