[KarlInOhio]

I thought one of the definitions of a separate species was that two different parent species could not produce fertile offspring. How could they co-mingle the 2 species?

If you could find evidence that either they could breed together or couldn't, then you would answer the question of whether they are separate species. We don't know for sure, so all else is speculation.

I don't think that genetic science has advanced far enough to tell just from DNA samples whether two close samples could interbreed or not.

[Wolfstar]

If you could find evidence that either they could breed together or couldn't, then you would answer the question of whether they are separate species.

This is incorrect. Closely related species potentially can interbreed successfully. The key word is potentially. Examples of closely related, but different species that can interbreed include (but are not limited to) wolves and dogs, foxes and dogs, horses and donkeys, lions and tigers, sheep and goats, bonobos and chimps.