Not all species reproduce sexually. Not all sexual reproduction involves pairs.

"A single Tuvix is a curiosity. A wonder, even. But thousands of Tuvixes -- isn't that becoming a race?"

Tertiary question: we know that Vulcans/Romulans were descendents of the Progenitors, but are Talaxians?

If not, it might not be technically possible for them to interbreed at all, meaning that a Vulcan/Talaxian hybrid could never occur naturally, and also that Tuvix would probably be genetically incompatible with everything and everyone. Therefore, no species - only an anomaly produced by a freak transporter accident.