You have an 8x8 midi interface, with the two synths attached. Connect the MIDI from the BCR into the next pair of in/outs of the 8x8. When the 8x8 is connected to the Mac via USB, you can then patch a MIDI source (BCR) via Pro tools into the correct MIDI track outputs, to send data to either the DW8000/MKS50, just like you would with note data from the keyboard or Pro Tools DAW.
The MOTU homepage suggests that you could use the ClockWorks console software for "comprehensive MIDI routing, merging and muting". So you should have the alternative to set up some routeing presets via ClockWorks, and after that, not need the Mac/Protools running at all - just enjoy playing (unless you want Protools involved, eg for recording MIDI.
Hope that helps, Bill