Player Multiple Video in sync and loop by OMXPlayer-sync
setup: copy or download files execute install.sh file in superuser mode
setup all pi in same network, with router or switcher (you need to setup network by hub or switcher)
format your USB media to exfat format
role=m --master device (can be used as single channel video loop player) l --slaver device audio_source=hdmi or local usb=1 --play video from USB media 0 --play video from /home/pi/
Thanks to original sync script from Simon Josi.