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 Apple's Aggregate Device Mode feature in Audio MIDI Setup, within the OS.
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.