Can I have both a Toronto Metropolitan University/George Brown OneCard and a George Brown Student ID card?

It is not necessary to get both cards. 

The Toronto Metropolitan University/George Brown Student OneCard is the Official George Brown Student ID Card and it will provide you to access both Toronto Metropolitan University and George Brown College services.

If you wish to get a separate George Brown Student ID Card, it can be purchased at the cost of $15 at any of the main campuses in the Library Learning Commons (Casa Loma, St. James, and Waterfront).