the sync_in and sync_out parameters of the Config functions will need to be used.
Wondering if you can just config each SMU to make use of digital line 1. And then use the digio.trigger[1].assert() to satisfy the digital event.
Also, check your firmware and update to the final release (1.4.2) for these non-A and non-B vintage models.
firmware for 2600 nonA and nonB