You may have to create the INPUT and OUTPUT busses in Protools and assign those inputs and outputs from the second MR there.
A work around would be to use ADAT cables and hook up the second unit to the first using ADAT optical. Only issue there would be your limit on sample rate 48/44.1 only. S/MUX compatible, 4 in and 4 out @ 192kHz as well though
Hit up member Norbury Brook as he daisy chains these as well