Two canaries in a coal mine?

I'm not sure the question here. The phrase of a "canary in a coal mine" refers to an early warning of disaster.

Note link added from Wikipedia:

By analogy, the term climate canary is used to refer to a species that is affected by an environmental danger prior to other species, thus serving as an early warning system for the other species with regard to the danger.[3]

Apparently, the phrase "two canaries ..." was used in the film, Inconvenient Truths, in identifying two early warnings of Global Warming.