The 32-year-old CEO of a $2 billion startup says he and his cofounder are driven by paranoia

Nat Turner met Zach Weinberg on their first day of school as undergrads at Wharton, in a class known for being the easiest way to fulfill the writing requirement ("

They hit it off, and one day after work one night they started talking about a mutual love of starting businesses. "And we started brainstorming," Turner, now 32, told us for an episode of Business Insider's podcast "Success! How I Did It."

"He's one of the best product and startup guys I know, to be honest," Turner said of Weinberg. "And even then he was so good at it."

That was 2004, and it was the start of a business partnership that would see an $80-million deal to Google in 2010 and a $1.9-billion deal in February for their current venture, Flatiron Health.

Flatiron develops software for cancer treatment centers that helps doctors better monitor patients' progress and easily share data with researchers. The deal earlier this year was made with the Swiss healthcare company Roche, and Turner will remain CEO.

He told us why he and Weinberg have been able to be business partners for nearly 15 years, from a college dorm hobby to multi-billion-dollar company.

"Zach and I, it's not all rosy and we certainly have our ups and downs, like any marriage," Turner said. "Zach and I like to joke to our wives that they're the second wives."

It's a matter of recognizing what the other is good at and letting their expertise in that area shine through. And while they have complementary skill sets, Turner said, they share a common drive.

"One thing Zach and I are, we're very paranoid. Which drives us, maybe to a fault." At their ad company Invite, "we were at the office most nights to 2:00 and 3:00 in the morning and most of the time it was doomsday scenarios like, 'What if this company does that? What if that company does this?'"

After selling Invite to Google in 2010, Turner and Weinberg became Google employees. It took them all of one day to realize they wanted to remain business partners.

"I remember we had a whiteboard at Google," Turner said. They gave us this little office — probably to contain us. Zach and I and this whiteboard. And we knew we were going to start another company. There was no question."

Their next project was Flatiron Health, which was inspired by Turner's younger cousin's ultimately successful battle with cancer. When they realized that there was a market opportunity to do something that also benefited society, they realized that would be their next project. As Turner said, both of them reached the conclusion that, "our obituary one day, when we die: We don't want to say we optimized banner ads online."

It's the shared mission that is now the foundation for their partnership.

Turner has grown into a manager who can run a rapidly growing team, and Weinberg envisions what's next around product design.

But the similarities they noticed as college freshmen is what's kept them working together. "We love competing. We love questioning things. We love shining light on things. We love building things."