Shanghai is 6 hours ahead of Barcelona
Best overlap: 8:00 AM to 12:00 PM in Barcelona is 2:00 PM to 6:00 PM in Shanghai.
8:00 AM to 12:00 PM in Barcelona is when Shanghai is also working, at 2:00 PM to 6:00 PM their time. Roughly 4 hours of common time means scheduling is manageable; just avoid the very edges of the window. Heads up: because Barcelona changes its clocks and Shanghai doesn't, the difference moves by an hour twice a year. Confirm the overlap around the DST switch.