So can I maintain residence in both countries?

If you are lucky enough to have a green card proving permanent residence in the United States and a maple leaf card (same as a green card in the U.S.A.) proving permanent residence in Canada, your goal should eventually be to become a citizen of one or both countries. In the meantime, however, you can honestly abide by the requirements of both countries if you maintain domicile in the United States but also ensure you are in Canada sufficiently over the course of each five years to maintain your Canadian status as well. The key to travel, however, will be how you present yourself on entering each country.