If you are using more then just one single MR-interface for your recording setup in a "stacked" combination, you´ll need to follow this special setup instruction to assure the no-latency monitoring abilities of the MR-interface. Of course it is possible to combine different types of MR interfaces, it makes no difference if you´re using a combination of two MR816 CSX and one MR816 X or just two MR816 X and one MR816 CSX. The following setup example consists of two MR816 CSX and one MR816 X to create a recording setup with 24 analogue inputs. 

Hardware Setup
First of all, please connect the Firewire ports of each single device.
In our example the MR816 X is the "base"-device that is connected to the DAW via Firewire as well. According to that, please connect the remaining devices as listed in the following scheme:

DAW -> MR816 X Port 1 
MR816X Port 2 -> MR816 CSX (1) Port 1
MR816 CSX (1) Port 2 -> MR816 CSX (2) Port 2  

Accordingly, please connect the S/PDIF jacks from top to bottom ("base"-MR):

MR816 CSX (2) S/PDIF out -> MR816 CSX (1) S/PDIF in
MR816 CSX (1) S/PDIF out -> MR816 X (base) S/PDIF in

Software Setup
Please open the MR-editor. Corresponding to the connection set for all MR interfaces, each MR interface should be listed and displayed in the MR-editor.
If not, please take a look whether the Yamaha Steinberg Firewire driver is installed and working properly. Please also make sure that the option "ADAT (1-6) + S/PDIF(coax)" is selected and set for each interface!

Furthermore, please configure the "base" MR as follows:
Go to the S/PDIF output section. Here, please set the channel faders 1-8 and the master fader to "0" dB. You can just take a click on the fader-button while pressing the "ctrl"-key and the fader will automatically snap to "0" dB. Please save these settings as a "scene" by using the "file"-button in the end!

Please also configure the remaining MR-interfaces:
Select the S/PDIF output section and set the channel faders 1-8 and the S/PDIF input channel to "0" dB. Please remember to save these settings, too!

Sequencer Setup
Now, please start Cubase/Nuendo and make sure that the "Yamaha Steinberg Firewire ASIO"-driver is selected as ASIO driver in your device setup. 
Then switch to your VST-connections and set up your analogue and digital inputs and outputs according to the recording setup that you want to have. Please remember that we need to configure the S/PDIF inputs in a special way to connect the MR interfaces. To rebuild our setup, please connect the digital S/PDIF inputs of the "stacked" MR interfaces as follows:

Now you´ll need to activate and configure the Control Room in Cubase/Nuendo. Please create a minimum of three Studio/Monitor sends and connect them according to this screenshot:

In this case, the port-assignment of Monitor send 3 matches with our real monitor mix output in the end! To guarantee a perfect signalflow through the devices, you´ll need to select the S/PDIF output of your "base" MR for Studio/Monitor 1 and the S/PDIF output of your second MR for Studio/Monitor 2.  
In this example we have selected some analogue outputs for the assignment of our headphone mix. If you want to use the MR headphone-amplifiers, just configure your headphone mix to these outputs with the audio hardware setup in Cubase/Nuendo.  
Now switch to your mixer in Cubase/Nuendo and create 24 mono inputs according to a 24 track analogue input recording. (This is just our example. If you want to add some digital ADAT inputs as well).

Please also add two additional stereo channels for our interface-routing and disperse the sends as follows:

Please configure the additional stereo channels as follows:
Activate the monitor buttons in both channels and set the channel faders to "0" dB.
Select the S/PDIF input of your "base" MR as input for your first channel and also activate the Monitor 2 in here. For the second channel, please select the S/PDIF input of the third "stacked" MR as channel input and activate the Studio/Monitor 3. The purpose of these additional channels is to integrate the signals of each interface into Cubase/Nuendo. You can see in this example that the signals from channel 8-16 supply Monitor 2. To add the signals from channels 1-8 to the sum, we need to use this first additional stereo channel. By activating the Studio/Monitor 2 in this "support" channel, we can achieve the signal transmission of the channels 1-8 to our monitoring sum.
Please do not change the settings of these two channels, otherwise changes will have a negative effect on your signalflow.

Please go to the contol room now:

In this setup, every single send signal will be summed in Studio/Monitor 3, so its sum is adjustable via Studio/Monitor 3 in the control room.
Please keep in mind to set the option "direct monitoring" in your device setup to enable no-latency monitoring. With this setting you´ll lose your level-meters in the control room.
This is just an exemplary setup for 24 analogue inputs/playback without any sum output. Please remember that it´s also possible to use the additional ADAT I/Os.