Eric Koger and Susan Gregg Koger carry their pug, Winston (pictured), to work. Often they bump into Kevin and Julia Hartz. Not a total coincidence: Their offices sit in the same building, in San Francisco’s South of Market neighborhood. The two couples share more than real estate: Both are running successful tech startups as a husband-and-wife team. The Kogers started ModCloth, a vintage-inspired clothing e-commerce site, in 2002 out of Susan’s dorm at Carnegie Mellon (she and Eric were dating; they married in 2006). In 2005, Julia moved to San Francisco to work on developing Eventbrite, an online ticketing company, with her then fiancé, Kevin.
Though they’re now well funded — ModCloth has raised $48 million and Eventbrite $140 million — early potential investors balked at husband-wife teams. “Nobody would give us the time of day,” Kevin says. The ModCloth pair also had their share of difficulty. In 2006 the Kogers bought a house in Pittsburgh (“because they were giving mortgages to everyone,” Eric says) and used tenants’ rent to help defray costs. They had backup plans. During Eventbrite’s early days the Hartzes created “out clauses” to avoid jeopardizing their personal relationship. They dreamed up worst-possible scenarios and their solutions; if it came to it, who would leave to get another job? The Kogers’ plan was looser: They decided that Eric, who had received his MBA from Carnegie Mellon, could always work in finance if they needed to cover the bills.
That kind of communication is equally important in their professional and personal lives. Both couples claim their skills are complementary to their spouses’, and they use each other as sounding boards. Eric says it’s nice to have a wife who understands the context of his happiness — or frustration: “When we brainstorm at home, you have a really active participant on the other side.” And the Hartzes praise their crew of married-founder friends for helping them structure their lives, including Bebo’s Michael and Xochi Birch and PopSugar’s Brian and Lisa Sugar. “It’s like a Tupperware party of wisdom,” Kevin says.
Susan on focus: “When you’re a founder, doing what’s right for [your personal life] is usually what’s right for the company.”
Eric on partnership: “You have a built-in stability. If you stay married, you’ll work together.”
Julia on partnership: “The scarier reality for us would be, what if we didn’t work together someday?”
Kevin on feedback: “It’s like I’m on truth serum with Julia. It’s very cathartic but is very enlightening and helps me solidify my thoughts.”
This story is from the September 16, 2013 issue of Fortune.