yes, i have two MOTU MTP AV USBs networked this way. On one end I have my main DAW which is a hackintosh, the other end is my MDD OS9 mac with a bunch of cards including a Korg OASYS. the two are also linked via ADAT. What's fun is that when you use it in this config, the machine at the other end of the chain effectively becomes a 17th midi port, and you can route data to and from the two machines, so i can for example use my hackintosh DAW to send clock sync and midi data to the OASYS., or my controller keyboard to trigger anything on the MDD.
ive recently been considering trying to do sequencing from the MDD since i learned on Studio Vision Pro and I think it would be fun to revisit.
way back in the day I also used that same setup as your friend, with a studio 128x networked to a studio 4. worked like a dream.
the emagic interfaces allowed you to network even more, I believe up to 8 units for 1024 channels.