Yes, you can connect two Quartet’s to your Mac and use the inputs and outputs of both units at the same time.  This is done through a process called aggregation.

 

Note: Both devices must be directly connected to an available USB port on the Mac.  You cannot daisy chain the second Quartet from the first Quartet’s USB MIDI port.  

 

See “Using 2 Quartet’s – Aggregate Device Setup” for more info.