I think there is no problem. As for my parents, they came to visit Canada this summber with single-trip tickts. After that, they went to U.S. and returned to China from U.S.
Where did your parents get the US visa, here in montreal or they got the visa before they came to Canada?
My father's brother is a PR of US and is now living there. My parents wish to visit them while they are in Canada, but they'd like to apply the US visa here in Montreal. Do they need some kind of relationship certification from my uncle in US? Or a Letter of Invitation is good enough?