Two years ago, Mr. Dixon, now 41, a certified public accountant and associate real estate broker for Douglas Elliman, joined forces with his friend Shane Hogan, an insurance broker, to buy a four-bedroom Cape Cod-style house in Sag Harbor, N.Y. They paid $950,000 for the property, which included a pool, and spent $150,000 on renovations.

“There’s something comforting about having a co-owner,” Mr. Dixon said. “If I could afford exactly what I wanted and a manager to manage it all, I would do it on my own. But there’s something nice about making the decisions with a friend.”

So far, he and Mr. Hogan have disagreed only about renting the property to help offset operating costs. “I tend to want to use the house more,” Mr. Dixon said. “Shane wants to rent it out for more income.” Their 10-page operating agreement comes in handy at times like this.

Such a document, referred to by some co-owners as a prenup, spells out the terms of engagement. For example: how bills are to be paid, how often friends are permitted to visit, how frequently one owner can have time at the house without the other owner. (“We can ask for 14 days up here alone, but none of those days can be on a holiday,” Ms. Hembree said.) And of course, there are the what-ifs: what if one party marries, has children, moves, has a reversal of fortune or just wants out.

But not every possible sticking point can be dealt with on paper and notarized. Some co-owners rely on their long friendship to come more casually to an amicable accord. When, for example, the three owners of the Cold Spring house are contemplating a furniture purchase, “our approach is that two of us have to agree and the third one can’t vehemently disagree, or we don’t buy it,” Ms. Jones said.

A decade ago, when David Waymire, Roger Martin and the men’s wives bought a condo at Boyne Mountain, a ski area in northern Michigan, scheduling was a problem. “But now we have a rhythm where there is a sit-down early in the year, when we work out the weekends we each want to be there,” said Mr. Waymire, who owns a public relations firm in Lansing, Mich., with Mr. Martin. “We’ve been close friends for 30 years, so we know how to deal with conflict.”

And they know how to deal with code. “There are some weekends that we’ll be there together,” Mr. Waymire said. “But sometimes Roger will mention something about a family weekend, and I know he means, ‘You can come if you want, but I’d rather you didn’t.’”