Looking to book YYZ-IAD-PEK. Sort of last minute, normally would fly YYZ-PEK, but going through IAD is significantly cheaper this time around and the savings cannot be ignored.

I'm a dual Canadian and US citizen. Chinese visa is in my Canadian passport. Can I check in with United with my Canadian passport and then present a US passport to CBP? I vaguely remember that scanning your passport at the CBP kiosk it automatically pulls up your flight info, since I won't be presenting the US passport to United, is that going to cause an issue?