The author is a Reuters Breakingviews columnist. The opinions expressed are her own.

By Jennifer Saba

NEW YORK, Oct 2 (Reuters Breakingviews) - It is an iron rule of corporate governance that one boss is almost always better than two. So it's striking that Oracle ORCL.N, Comcast CMCSA.O and Spotify have recently decided to install dual chief executives. Companies from Citi C.N to Chipotle Mexican Grill CMG.N have learned the hard way that such arrangements invite power struggles, dilute authority, and inflate wages.

Oracle kicked off the latest vogue for crowded corner offices when it promoted Clay Magouyrk and Mike Sicilia to become co-CEOs. They replace Safra Catz, who is sticking around as executive vice chair, alongside Chairman Larry Ellison. The $800 billion software giant justified doubling the number of senior decision-makers because of its quest to become a leader in artificial intelligence.

Cable and media firm Comcast joined in by elevating President Mike Cavanagh to co-CEO, alongside Brian Roberts, who also serves as chair. Meanwhile streaming music firm Spotify Technology SPOT.N split the top job by naming Gustav Söderström and Alex Norström to replace Daniel Ek, who is moving up to executive chair.

None of the newly elevated CEOs will have untrammeled power over the companies they lead. The 81-year-old Ellison remains firmly in charge of Oracle. Comcast's Roberts has outsized influence through his control of the $114 billion company's super-voting shares. Meanwhile, Spotify's Ek will retain very CEO-like duties, including determining capital allocation, mapping the $143 billion company's long-term future and supporting the senior team.

Netflix NFLX.O provides some positive inspiration. Ted Sarandos, known in Hollywood creative circles, and Greg Peters, a seasoned business operator, share the top job with founder Reed Hastings as chair. The video-streaming giant’s shares have nearly quadrupled since Netflix appointed co-CEOs over five years ago. Similarly, Neil Blumenthal and Dave Gilboa share the corner office at eyewear group Warby Parker WRBY.N, whose stock has risen 70% over the past twelve months.

Yet these are outliers. The 87 public companies led by co-CEOs generated an average return of 10%, barely above 7% for each of those companies’ relevant indexes, according to research conducted by consulting firm Feigen Advisors. And only 1% of companies in the Russell 3000 Index have such an organizational structure, per Equilar.

The dearth of co-leadership owes much to past high-profile flops. When Citibank merged with Travelers in 1998, their respective leaders, John Reed and Sandy Weill shared duties running the financial colossus. Their partnership was marred by turf wars and power grabs before Reed abruptly retired two years later. Software group Salesforce's CRM.N experiment ended badly with both Bret Taylor and Keith Block decamping elsewhere. Whole Foods co-CEOs Walter Robb and John Mackey came under scrutiny for overcharging customers before scrapping the six-year trial.

Perhaps the most powerful argument for picking one CEO comes down to compensation. Dividing up the duty often doubles the cost. When Peters took the reins as co-CEO of Netflix, his pay package increased 50% to $60 million, matching that of his partner Sarandos. Two-headed bosses deserve to remain rare beasts.

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CONTEXT NEWS

Spotify announced on September 30 that founder and Chief Executive Daniel Ek will become the music streaming company's executive chair. Spotify promoted Gustav Söderström and Alex Norström to be co-CEOs.

Comcast said on September 29 President Michael Cavanagh will share the role of chief executive with Brian Roberts.

Oracle on September 22 said Chief Executive Safra Catz would become executive vice chair and promoted Clay Magouyrk and Mike Sicilia to be co-CEOs.

(Editing by Peter Thal Larsen; Production by Pranav Kiran)

((For previous columns by the author, Reuters customers can click on SABA/jennifer.saba@thomsonreuters.com))