Ever since revision F1, the 1A2 seriss boards could be linked
together with a ribbon cable so as to manage 4 CO lines and 8
separate extensions that can each be dialed on the intercom line
by dialing "1" thru "8".
The general docs for interlinking cards can be seen in the "Expansion" chapter of the User Manual, which in the case of that REV-J5 manual, it starts on page 19. Or, this single page diagram showing the simple connection notes: http://seriss.com/1a2-ksu/rev-j5/data/1a2-REV-J5-0015.png
Some actual hook up notes: I suggest mounting both boards to plywood back board about 2" apart on rubber spacers, horizontally aligned, like this:
This way the ribbon cable can be run underneath the boards, so
it's out of the way - folded "under" the boards with bends (but no
The new REV-J boards also have slots to allow one to utilize velcro straps (optional) to retain the connectors.
To increase compression (if the holes are too large), just add a thin strip of e.g. felt or rubber, or other flexible material between the boards, then tighten down the wing nuts to grip the cables.