Unfortunately, this time your NRMA lady is correct. The 'Carnet de Passages en Douane' is definitely only valid for one vehicle.
The confusion probably arises because there is a commercial/business carnet issued by ATA. This would be what your State Chamber of Commerce is referring to.
An ATA Carnet covers business goods taken to other countries, as distinct from the 'Carnet de Passage en Douanes', which covers PERSONALLY owned vehicles and is issued by the automobile associations.
For a detailed explanation of the difference between these, see:
But, the 'official' word from the Canadian Chamber of Commerce is:
<font face="Verdana, Arial, Helvetica" size="2">"... vehicles are only allowed on an ATA Carnet for racing (and the like) purposes and/or if part of professional equipment, i.e. mobile unit used by TV crews and the like. Vehicles for personal use should be registered via the Canadian Automotive Association (CAA)."</font>
Now, you may think, well we can pretend we're a professional film crew or racing crew, but the problem is that you then are asserting that you're travelling on business, which may cause problems if your visa says you're supposed to be tourists only.
But the good news is that you don't need a carnet at all for the Americas, north or south. Although a carnet will simplify customs clearance, it is NOT required. So, unless your trip will also involve either Africa or Asia, don't worry about it.
'One world, two wheels'