Zurich is 7 hours behind Shenzhen
Best overlap: 3:00 PM to 6:00 PM in Shenzhen is 8:00 AM to 11:00 AM in Zurich.
Both offices are open during 3:00 PM to 6:00 PM Shenzhen time, which is 8:00 AM to 11:00 AM in Zurich. You've got a decent 3-hour window. Set a recurring time near 3:00 PM in Shenzhen (8:00 AM in Zurich) and stick to it. Just remember Zurich changes its clocks for daylight saving and Shenzhen doesn't, so confirm the overlap whenever the seasons turn.