If you want to solo more than one track at the same time, just select multiple tracks with the Ctrl-key. Those tracks will all play with the solo button pressed.
I have suggested that before (see pics), this is just to show that BB has solo mute buttons but RB don't, in BB you can only solo one track at a time, so when they put it in RB you would need to be able to solo more than one track at a time. In RB at the moment it is a pain in the ass soloing the tracks you need to hear and them staying soloed especially if you are working with a hell of a lot of tracks.