just plug one unit into your computer with the FW cable and then plug that units second fw port into the second unit,they automatically assign themselves as MR816(1) and MR816 (2) or (0) and (1) can’t remember off hand.
Bear in mind that ONLY the first unit will have zero latency direct monitoring,so if yo want 16 analogue ins with DM you’ll have to use the second 816 as a slave into the first via ADAT; so the second 816’s pre’s output into the first 816’s ADAT ins . The only issue with this method is that the second unit will not follow sample rate changes automatically so if you work at different sample rates it’s a PIA as you’ll have to keep plugging the unit in separately with a FW cable and manually change the slave sample rate. This is going to be addressed in the next firmware update however.
To set up a master slave like this you need to plug both units in separately to access the MR editor,when daisy chained you cannot adjust the master slave settings discretely.