Extend 6 month visitor stay?

[Reba]

Once again, Canada does not care if you overstayed in the US. Only the US will care.

[Steven]

Does Canada and the U.S share information at border crossing.

Yes they do, I described an experience at YYC earlier in this thread where they didn't (or rather they tried and they couldn't), but I got pulled up by CBSA at Sweetgrass about a month ago and they could see my CBP entry record.

So basically, yes they do share information but like any IT system it has its moments, looks like.