create your main track as normal
drop your audio track into a2 in the same track
1 button on your main menu will be pointed to the main track
create a 2nd button...go to inspector...make sure the target indicates the same main track...go to the advanced tab...under audio...choose a2
that should do it.
it's been awhile since i've done it, but i think that is it...if i goofed..it's not far off the case 🙂