The first "two-cat" systems I knew of were on GM pickup/Suburban vehicles. The main cat (which was for HC and CO) added an extra cat near the rear axle for NOx reduction. When the main cat would overheat (high loads, as in horse trailer towing, for extended periods) would cause the front bead converter to come apart internally. The resultant release of the ceramic beads from their "holder" would then go into the muffler and then back to the rear NOx converter, which was had ceramic honeycomb guts. When that rear converter got clogged with the beads, the vehicle was prone to stopping where ever it was a that time. Sometimes in the middle of a freeway with the 12-horse horse trailer attached. Not Good. Ford had a similar system, too.
The front converter would get so hot, it would cook the carpet in the rh side of the cab, through about 3" of padding! So, the warranty fix was new factory carpet, a y-pipe back exhaust system (which included all of the welded-in converters), and other related items. As our dealer-owner was active in the National Cutting Horse Association, we saw lots of HD2500 "horse hauler" rigs with those failure modes.
For cars, there was ONE cat converter which had TWO internal sections. One ceramic was for HC & CO, with a rear section ceramic for NOx. Looked just like the prior HC & CO converters, but usually had an extra air injection pipe (attached to the normal AIR pump system) going to it.
Canada, then being a separate country as now, plus the European Union (and related) countries, each did things differently. Then as now. Related to vehicle emissions and fuel economy issues. Had it not been for cimate and atmospheric pollution interactions in 1940s California, the California Air Resources Board would not have been configured and have gained a waiver (from the USA Clean Air Act, which was signed by President Nixon) to have automotive emissions standards more stringent than the rest of the USA. As things progressed into later decades, other states decided to follow the California standards.
From what I could see, Canadian emissions vehicles were USA Federal-spec emissions vehicles without the cat converters on them, mostly.
Enjoy!
CBODY67