I use the general midi channel on the Tempest (default 10) and the pad assignment notes to send beat changes to the Tempest from the Octatrack. You have to nudge the OT trig a few notches before the grid for the everything but the first pattern (ie: to the left) to have the Tempest beat start on time when changing OT patterns. I've only recently switched to this method for live use, but so far it seems pretty sweet.