The versions have to be the SAME.
Versions 1 and up can be interconnected,
even some of the pre -1 units can be.
2nd cabinet has to be set to slave.
Version 6 and later have "auto-sync"
The version 7 is actually the same software version as the 6 it's just that the -7 comes with a pre installed 193 CID card.
It is still recommended that both cabinets be brought up at the same time with NO activity at all for 3 minutes.