Can multiple Speaker Set buttons be selected simultaneously with Quartet?

Yes!  You can listen through multiple speaker sets simultaneously.  Here's how:

1. In the Output tab of Maestro, change the main output selection to "2 Speaker Sets" or "3 Speaker Sets".  The A B C speaker set selection buttons in Maestro will become active.
2. Command (Apple)+click each speaker set button you'd like to select.  You can freely select some or all of the buttons to be on or off a the same time.  For example, if you'd like to simultaneously listen to Speaker Sets A and B, first click the "A" selection button in Maestro, then Command click the "B" selection button.

Note: Selecting multiple simultaneous speaker sets can only be done in Maestro.  You cannot use Quartet's assignable Touchpads to select multiple speaker sets.