I ordered one from Sweetwater the other day(apparently, I'm the 2nd to order one, ha!). It appears that it will simply operate like any other Avid HD I/O via Digilink. I have an HDX2 setup and the first card has both Digilink ports being used by my MTRX Studio, with both Digilink ports on the 2nd card free. I'm hoping it's as simple as plugging the Digilink from the PT HD Expansion card into the primary port on my second HDX card. I guess I will also use the Loop Sync to connect MTRX Studio to the PT HD Expansion card, but not sure if it's completely necessary, since I have the word clock already connected from the MTRX Studio to my H9000R (Where it terminates. Kudos to Eventide for including the terminator, btw!).
I think the setup you suggest should be fine.
If you're using a central word clock connected to both the MTRX and H9000R, you shouldn't need to connect the loop sync.