Good question ... but a little tricky if you want to be REALLY technical!
The answer you're looking for is John Huarte and Matt Leinart who both played quarterback for Mater Dei in Santa Ana, California.

That said, on a technicality, the only other high school to produce two Heisman winners is also the correct answer - that being Davey O'Brien and Tim Brown who played for Woodrow Wilson High School in Dallas. The technicality is that O'Brien was a quarterback (hence the Davey O'Brien award - given for the best QB in collegiate football) but Timmy Brown, on the other hand, played a myriad of positions. Though he was primarily a defensive back in high school, Tim also had played the quarterback position which, technically, makes the pair eligible for the correct answer to your question.

Peace!