Shenzhen is 7 hours ahead of Barcelona
Best overlap: 8:00 AM to 11:00 AM in Barcelona is 3:00 PM to 6:00 PM in Shenzhen.
Both offices are open during 8:00 AM to 11:00 AM Barcelona time, which is 3:00 PM to 6:00 PM in Shenzhen. You've got a decent 3-hour window. Set a recurring time near 8:00 AM in Barcelona (3:00 PM in Shenzhen) and stick to it. The window above shifts seasonally: Barcelona keeps daylight saving while Shenzhen ignores it.