US Federal law requires you to keep the same number of catalytic converters, in the same location as stock--unless you've got an exemption number from the EPA, or from the California Air Resources Board (CARB). Canada generally follows US Federal emissions laws, but there may be differences I'm not aware of.
If your vehicle was made with one catalyst, you can't install two.
If your vehicle was made with two catalysts on the passenger side, you can't move one to the driver's side, or sneak it in under the transmission.
You cannot remove a FUNCTIONING OEM catalytic converter even if you install an aftermarket "high performance" catalyst in it's place.
It is legal to remove a non-functioning OEM catalyst in order to install an aftermarket "universal" catalyst...but not in California.
Screwing with the EPA is a FEDERAL offense--the Feds can (but usually don't) bust you anywhere in the USA. Point is, if they want you...you've given them a way to get you.
CARB, of course is applicable only in California, and other states that have adopted Crazy California emissions requirements, so those states may require CARB exemptions, or perhaps they have their own emissions-compliance officers. Emissions regulations can be more strict than EPA, but never LESS strict. Under CARB, "universal" catalysts are likely illegal--you have to have "OEM-quality" catalysts which may or may not be made/sold by the vehicle manufacturer, but they're about three+ times as expensive as the "universal" cats legal in the other states.
A business faces serious fines for violating EPA regulations. An individual so far as I know doesn't face massive fines for DIY violation of the EPA regulations, but the vehicle registration can be revoked unless it's "repaired". Again, enforcement is sparse and inconsistent, but it can exist.
BE AWARE of what the Government can do to you when you make them unhappy.