You can declare one version as canonical, but then you'll be influencing any test to see which one works better. If you just publish both versions without canonicalizing, the search engines will canonicalize themselves. That is, they'll decide which version to count and which to ignore, which I guess will answer your question.
Are you asking about putting an entire site on two different domains, or experimenting to find out which of two different sites is the better place to publish one particular piece of content? If we're talking about two copies of the same site on two domains with no history or links, the whole thing might end up being random and telling you nothing. The search engine will likely treat the one it finds first as the canonical version. And if that's the case, it's probably a waste of time to test this. Just pick the domain name that you feel is going to be easier to brand for the kind of content you plan to publish and forget about the other.
And if we're talking about two sites that don't have the same history, one of them is likely to be stronger than the other based on some signal or signals the search engines look at, and it's going to win your little contest, but not because it somehow works better for this content.
If this is about deciding which of two different sites should get this one piece of content, I think you're better off thinking about the different audiences (assuming there's some difference between the audiences) and basing your decision on that rather than something algorithmic, in part because if you leave something like that up to the search engines, they're likely to pick the site with a stronger information architecture -- the one that's structured and linked in such a way that the new page is going to start off a little stronger.
In any case, once you decide which version to keep, get rid of the other and set up 301 redirects from it to the winner.
On edit, long story short... what Chris said.
Edited by qwerty, 28 July 2014 - 04:34 PM.