The following applies to Mac OS X 10.4.11 and higher operating systems.

Install the Allen & Heath FireWire driver following the procedure in the user guide.

Ensure the switches on the back of the ZED-R16s are set to FireWire.
Connect each mixer individually to the computer, again following the installation procedure.
It is a good idea to give each mixer a unique name by using the Change Device NickName feature in the driver control panel.
 

Now with both ZED-R16s powered but disconnected, connect the first ZED-R16 to the computer.
Wait for a few seconds to allow the system to establish, then connect the second ZED-R16 to the first one using the second FireWire port.

Open up the driver control panel from the Application folder and check that both ZED-R16 devices are connected and locked.
This can be checked by switching between the available devices.

If either of the devices is not locked, try switching the Master Device between the two different mixers and then rechecking that the available devices are both locked in the Status box.

The ZED-R16 with the lowest device Unique ID will be set up with CH 1 to 26.
The channel settings will be:

Mixer 1 Inputs 1-16 to FireWire channels 1-16
Mixer 1 Master LR to FireWire channels 17-18
Mixer 1 ADAT 1-8 to FireWire channels 19-26

Mixer 2 Inputs 1-16 to FireWire channels 27-42
Mixer 2 Master LR to FireWire channels 43-44
Mixer 2 ADAT 1-8 to FireWire channels 45-52

When disconnecting the desks it is important that the desk connected furthest away from the Mac is switched off first
followed by the desk connected to the Mac.

For subsequent power-up in this setup, power the mixer connected to the computer first, waiting a few seconds, then power the second mixer.