The key thing, from a legal standpoint, is that you do know that you've got an extra TV that you haven't paid for. I would say that it wasn't a great idea to acknowledge that fact in a public internet forum, though I'll admit that it's unlikely that such a post could ever be used as an admission on your part. Just note that theft is the intentional or knowing appropriation of someone else's property. You know that you have their property, and if you don't return it, it could be said that you're intentionally in possession of their property without a lawful authorization. In practical effect, you're likely to be given an opportunity to return it if the mistake is discovered, which negates any criminal concerns with the transaction.
But since your original question was for legal advice, I feel fairly comfortable telling you that keeping the second television is probably a violation of the Texas Penal Code and could be prosecuted if discovered. That's not a comment on how likely prosecution might be, just an acknowledgment that it is possible.