A startup founder who built a side business while working in finance shares his best entrepreneurship advice

While they don't publicly share subscriber numbers, Martell says the newsletter reaches "six figures of eyeballs," and that it's been generating revenue for about three years, through syndication deals and promotions with brands such as Fidelity and Betterment. The founders appear regularly on TV and online video platforms such as Cheddar, Nasdaq, and CBS.

In a conversation with Business Insider, Martell said that he and Kramer prioritize serving as a resource for other entrepreneurs. They get on the phone with entrepreneurs seeking advice "every week or so," he said, and some of their go-to advice is based on a key component of their own business' success: their working relationship.

"Jack and I are complementary in a really fun way," Martell said.

That's why they advise other entrepreneurs to seek out "complementary cofounders" who have different skills and strengths.

For instance, though they both worked in finance, Martell and Kramer's expertise lies in different areas, and they "realized that there is power in playing to each others' strengths," Martell said. "When it comes to consumer goods companies like Nike, or Lululemon, or Chipotle, I tend to write about those because I know those areas well. And when it comes to industrial or energy stocks like Boeing or Exxon, Jack writes about those."

Also, "I'm a little more risk-oriented than Jack is — he's a little more conservative in his approach," Martell said. That's a good thing, he explained "because there's times when I'll, for example, want to jump into something and Jack will want to take a more traditional route, and whichever one kind of feels more strongly about it, we tend to back and go behind."

He continued on to say "We always advise entrepreneurs: If they're going to start a business, having a complementary cofounder is so key because it can help you go into new areas really quickly in a really smart way."

Over the course of their business' life, Martell spent four years traveling internationally for his job and now spends most of the week pursuing an MBA at Wharton in Philadelphia. Kramer had been working in Berlin and now is pursuing an MBA at the University of Michigan, and the two founders are rarely in the same time zone. Aside from TV appearances in New York City, they work almost entirely remotely.

Although that sounds like a challenge, they used their complementary skills to create a system "to be as efficient as possible so that we can write 750 words a night but still have balanced lives with our day jobs and our social lives, but also scale the business," Martell said.