Two hours before Microsoft’s Mac software group announced an online collaboration tool for Mac Office users this week, it got a surprise: Apple had been working on something very similar for its own Mac word-processing and spreadsheet programs.

And Apple was unveiling its tool first.

Welcome to one of the tech industry’s most complicated relationships. Despite the intense rivalry between Macs and Windows PCs, and increasing competition in Mac productivity software, the new head of Microsoft’s Mac Business Unit said this week that he feels good about his group’s bond with Apple.

“I think it’s stronger than it’s ever been,” said Eric Wilfrid, a veteran of Microsoft’s Mac Business Unit who took over in October as general manager. “There’s this huge mutual interest in taking care of our customers who choose a Mac for their desktop or laptop, and Office for their productivity software. That lets us really focus and work together really well.”

It looks quite different from the outside. Most notably, Apple in recent years has rolled out its own productivity suite, iWork, as a competitor to Office for Mac. It includes the Pages word processor, Numbers spreadsheet program and Keynote presentation software.

But the dominance of Word, Excel and other Office programs made by Microsoft still makes them critical software applications for most Mac users. For example, Microsoft said this week that more than three-quarters of Mac users in the U.S. are running the Mac Office programs.

“Office is a de facto standard,” said Guy Kawasaki, a venture capitalist and former marketer at Apple. In many corporations, he said, “No one would buy a Macintosh if there weren’t Office.”

At the same time, the people in Microsoft’s Mac Business unit have a vested interest in the Mac because it’s key to their existence. And while the group is a relatively small portion of Microsoft overall, because of the Mac’s relatively small share of the personal computer market, its business is profitable and has been growing. Considering all those things, it’s not surprising that Apple and Microsoft’s Mac unit would find ways to work together.

“They both need each other,” Kawasaki said.

For now, at least, that appears enough to overcome the larger rivalry between the companies. That competition has come to a head in recent years as Apple has lampooned Microsoft Windows — personified by a pudgy nerd — in a series of popular Mac vs. PC ads on TV and the web.

Apple makes subtle jabs, as well. Speaking at the Macworld Expo here in San Francisco this week, Apple marketing executive Phil Schiller said there was nothing nearly as good as the company’s iLife software suite — despite what some “people up north” might think.

Behind the scenes, the situation is much more cordial between Apple and Microsoft’s Mac unit, Wilfrid said. People from the companies meet every week to talk about technical issues, he said. They also meet periodically to discuss longer-term issues, such as what Microsoft can do to take advantage of improvements in Apple’s Mac OS X operating system.

However, Apple keeps some things close to the vest. For example, Microsoft wasn’t aware in advance that Apple would be unveiling the iWork.com collaboration service this week at the Macworld Expo, Wilfrid said. Released in preliminary form this week, it’s designed to let Pages, Numbers and Keynote users share and work together on documents online.

Features include document uploading, sharing, comments and notes. It will work with iWork, Microsoft Office and PDF formats, Apple said. Users will click on an iWork.com icon in the iWork desktop applications to sign in to the browser-based service using their Apple IDs.

For its part, Microsoft this week announced plans for a free “Document Collaboration Companion” for Mac Office 2008. The new companion service will let people share documents through the company's SharePoint and Office Live Workspace services.

It’s expected to be released in final form later this year.

Despite the similarities — and the surprise — Wilfrid took Apple’s move in stride.

Apple, he said, is simply “looking at some of the same user needs that Microsoft is looking at."