Dallas is 13 hours behind Beijing
There is no standard 8-to-6 overlap. Someone joins early or late.
Because Dallas is 13 hours behind Beijing, the two offices are essentially never open at the same time. If a live call is essential, aim for the very start or very end of a workday in one city and accept it'll be off-hours for the other. Don't be caught out by clock changes — Dallas observes daylight saving and Beijing doesn't, so the offset varies by an hour through the year.