Shenzhen is 7 hours ahead of Warsaw
Best overlap: 8:00 AM to 11:00 AM in Warsaw is 3:00 PM to 6:00 PM in Shenzhen.
Both offices are open during 8:00 AM to 11:00 AM Warsaw time, which is 3:00 PM to 6:00 PM in Shenzhen. Roughly 3 hours of common time means scheduling is manageable; just avoid the very edges of the window. Just remember Warsaw changes its clocks for daylight saving and Shenzhen doesn't, so confirm the overlap whenever the seasons turn.