You qualified your statement by using the word "practically," but the two citizen parent definition of natural born citizen (granted - outside of the law) existed prior to the founding of the Constitution.
Then it shouldn't be too hard to find usages of it. We have over 230 years of American jurisprudence to look at. So, if 'two citizen parents' or 'no dual citizenships at birth' or some combination of the two are the standard definition of NBC, then there should be no shortage whatsoever of usages of those definitions.
Those definitions should then crop up in textbooks and lawbooks and law review articles. They should have been applied to previous Presidential candidates who were the children of immigrants (often two immigrants), like Nader and Dukakis and Bill Richardson and Hubert Humphrey and Arlen Specter, and not just suddenly appearing out of the ether in 2008 to be used against a candidate born to an American mother. If you called up a Constitution Law professor at a law school, then they'd tell you "Why yes, that's a perfectly viable and accepted definition." It should be everywhere, not buried in the occasional 1800s legal dissent or obscure article.
Plus, considering that Obama's father's identity was never in dispute, and Obama didn't seal British citizenship laws from inspection, then you might imagine that a valid argument against his citizenship might have been advanced at some point during the first 16 months of his candidacy, rather than suddenly cropping up during the last 4. But it didn't.
None of that happened. In fact, the advancement of the theory didn't even happen during the earliest days of the allegations over Obama's eligibility; it was only once the holes in the 'born in Kenya' theory were apparent that some people began to adopt alternative theories for why he was *still* ineligible even though he was born in Hawaii.