Is there any legal way to fly an inline twin-engine aircraft with a single-engine certificate?
Under United States Part 103 ultralight category, with no license or aircraft certification requirements as long as the the limitations of the category are met (dry weight, fuel capacity, single seat, and level flight maximum speed, permitted flight locations), you can do anything you like.  Whether it's safe or a good idea is up to the pilot.

Last time I checked, Part 103 requires a dry weight (with engine, no fuel, no pilot) under 254 lb, no more than 5 US gallons (though actually called out in liters) fuel capacity, and level flight speed limit of 55 kt (about 62 mph).

I can't answer relative to Canada, as their regulations concerning ultralight/microlight aircraft differ from those in the USA.