If I have two passports, and use an American one to enter the USA and?
You tell them at the border basically. Plus they do track things like license plates so they can track you. Canada and the USA do share crossing data and they can link it up. The only reason for time out of country to matter for a dual citizen is determining if you need to pay duty on anything brought across. How long youve been out of either country beyond that is largely pointless since as a citizen of either country youre permitted to enter at-will for any duration you wish.