The behavior you see is normal. The frame sync output is always active on the master camera, even when the camera is not recording. To sync the two cameras properly, you'll need to set up a second trigger as a record end trigger, so that both the master and slave cameras stop recording simultaneously. I would also recommend setting the record length to the same value on both cameras, so that the recordings are of the same length are lined up properly. Otherwise you'll need to sync the two recordings based on the end rather than the beginning.
You'll want to turn off the 20mA pullup on the slave camera, to avoid possible issues with signal levels.
Let me know if this works for you.
Regards,
David