I think Amazon has already selected Toronto (or possibly somewhere else in Ontario like Kitchener-Waterloo or Windsor) and is just running this 'contest' to extract the best deal they can get. I don't see any reason for dual HQs unless they are hedging against the risk they may have to abandon their current HQ and the biggest threat they face is anti-monopoly actions by a hostile government. Any secondary HQ is going to be outside of the USA and they've already eliminated Europe and Asia.
No city in the USA can compete with Toronto on the basis of legal risk, cheap labour (senior software devs top out around $90k usd in Toronto) and population/infrastructure
A huge one: Seattle is starting to feel a bit like a company town. This has all sorts of practical considerations, like labor cost and real estate availability.
For example, from https://www.geekwire.com/2017/how-seattles-office-market-bec...: Amazon "takes up approximately 20 percent of Seattle’s office inventory, one of the highest concentrations in the country by a single corporate entity," and Amazon anticipates 50% growth in square footage between 2016 and 2020.
tl;dr: the strongest reasons for 2 HQs aren't political.
No city in the USA can compete with Toronto on the basis of legal risk, cheap labour (senior software devs top out around $90k usd in Toronto) and population/infrastructure