Next board iteration should have a standard ribbon cable connector on (right or back) side that passes through a synchronization signal that is only enabled when the master system is acquiring data.
The slave systems should work as usual. Boards should be set to slave mode with a OkWireIn or equivalent via switch in the rhythm interface UI. Linking can be done with a ribbon cable with one twisted pair on the clk pins on the master side.

Pins:

• GND
• sync out - use to start and stop slave systems
• sync in (out and in switched on master)
• TTL ch8 out connected to D out  - use to send periodic sync codes every N seconds just in case we get worried about tiny clock drifts
• TTL ch8 in, also just pulled from the D out channel (again, twist the pair on the master side so that we can send sync codes)