Portland is 3 hours behind Montreal
Best overlap: 11:00 AM to 6:00 PM in Montreal is 8:00 AM to 3:00 PM in Portland.
Both offices are open during 11:00 AM to 6:00 PM Montreal time, which is 8:00 AM to 3:00 PM in Portland. The 3-hour gap barely gets in the way — you've got 7 hours of overlap, so pick whatever time fits the agenda.