Karachi is 13 hours ahead of Portland
There is no standard 8-to-6 overlap. Someone joins early or late.
Because Karachi is 13 hours ahead of Portland, 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. Note that Portland follows daylight saving while Karachi stays put, so the times above drift by an hour seasonally.