Alright, starting with the simplest possible thing (MC sending to MS):
Two patterns A01 and A02, 16 step pattern both machines scale set up as Mod PTN, Len 16, Chg 16. Also Chain A01-A01-A02-A02
Everything works
Change receiver (MS) pattern A01 to Mod TRK … immediately things are broken. Change A01 to A02 is one cycle (16 steps) late on the receiver. Though change A02 -> A01 (with A02 on the receiver still set to pattern) the changes are in sync.
Assuming that’s the only thing that went wrong (which is a big assumption) we have one big rule already:
DON’T use Mode TRK on the scale setup of the RECEIVER.
Glad I followed your advice and started simple @thermionic 