US and Canadian customs are notoriously rigorous (to the point of absurdity) on the $10k limit: most customs officials seem to believe that carrying large amounts of cash (under 10k too) is inherently suspicious.

I understand from OP that this is a theoretical question - still, past experience has shown that people travelling together with over $10k cumulative have frequently run into problems, including seizure, unless it was demonstrated- and provable - that each individual personally owned the amount they were actually carrying.