how to use START/STOP ALL on 2 loopers

Ive just picked up another Boss R300 and followed the directions of this video: https://www.youtube.com/watch?v=T7j4O8N64kA

i was wondering if anyone knows how to sync the start and stop function from the master pedal to work on the slave pedal too. i know it can be done ive seen a person perform and do it live. any help is appreciated