Here’s a workaround:
Connect your second MR to the computer and set it up as standalone, all inputs routed to adat outs and all adat ins routed to analog outs and hook up this to your first unit with two lightpipe cables.Assign the two headphones to, say, the first two stereo analog outs
Disconnect your second MR and connect your first.
Now, you should see your second inputs coming in on your main adat ins.
You can use your 2 headphone outs on your main unit as usual and use 2 (stereo) adat out pairs (on main) to send to your second unit, then assign those 2 pairs to the second unit’s headphone outs.
This way you can use direct monitoring and send 4 independent phones mixes.