It's one form per address as far as I know. If you tell AC you are at different addresses you get two forms.

As to the legality of that however... I'm not sure. One assumes CBSA could, if they wish, check to see if multiple people on the same flight number on the same day with the same address each did their own form.

Incidently, if you declare and have a reason for over $10,000 I don't believe it's an issue. Or just transfer it via banks (again not an issue though there may be a question or two due to money laundering laws).