Even though they are two of the Internet’s largest advertising businesses, both Yahoo and Microsoft are without top execs to lead those units.

Worse, both are just entering a complex online ad sales and search partnership together, which will require a lot of management firepower.

Last week, BoomTown reported the departure of Yahoo’s head for the key North American market, Joanne Bradford, for a new job at social media start-up Demand Media. She starts there Monday.

It was a move that sent reverberations throughout the online ad market.

Less known, though, is that Microsoft (MSFT) has also been looking for almost a year for someone to head up its online ad sales force globally.

Yahoo (YHOO) is just starting its search to replace Bradford–with her boss, U.S. head Hilary Schneider, taking over on an interim basis.

In fact, even Yahoo CEO Carol Bartz is pitching in, giving Bradford’s staff a talking to earlier this week about the need to press on.

Both Yahoo execs said the company will be looking at both internal and external candidates.

Until then, said a Yahoo spokeswoman, in the boringest quote ever uttered: “Yahoo! has leadership bench strength and we continue to be committed in delivering wow experiences to both users and advertisers.”

Not to worry about obfuscation, as BoomTown has the scoop!

Internally, the key execs being eyeballed include 11-year Yahoo veteran Mitch Spolan, VP of North American sales, and Seth Dallaire, a former Microsoft exec whom Bradford brought to the company last fall as VP of mid-market sales, a newly-created role responsible for all mid-market sales efforts across search and display advertising.

Another former Microsoft exec, Erika Nardini, VP of brand packaging, is also mentioned a lot as a possibility and is well-liked by the sales force at Yahoo.

Still, many feel that with the exodus of such a high-profile exec as Bradford, Yahoo has to attract another big name to replace her.

But externally, the pickings are much slimmer, with only two key names popping up as top choices.

One is a former Yahoo, Jacki Kelley, a longtime online ad exec who is now North American president of Universal McCann, a unit of the Interpublic Group (IPG) agency. Besides Yahoo, Kelley has worked at Gannett’s (GCI) USA Today and Martha Stewart Living Omnimedia (MSO).

The second is Kathy Kayse, a well-regarded former AOL (AOL) ad exec, who now is in charge of digital ad sales at Discovery Communications (DISCA). Kayse also had a long career at Time Warner (TWX).

(Both are pictured here, Kelley at left and Kayse on the right.)

Microsoft is a dicier proposition, with exactly zero internal candidates considered qualified to lead the online ad sales effort, a job that would report directly into Corporate VP for Consumer & Online Darren Huston.

Microsoft has been searching for a long time now, with feelers all over the industry. A variety of names pops up, from new MySpace ad head Nada Stirratt to Bradford–also a former Microsoftie–herself.

In fact, the paucity of experienced execs to handle these complex jobs–which include the need to understand premium, network and search ad sales, as well as highly technical systems–is clear.

“Neither of these jobs are easy and, in many ways, a giant nightmare,” joked one online ad sales exec.

She has apparently grabbed Vice President of Sales Strategy Jacki Kelley (pictured here), who one Yahoo wrote me “was one of the good ones.” It’s unclear what role Kelley will have at Martha Stewart Living Omnimedia, where Millard landed this summer as its president of media.

[UPDATE: Kelley will be EVP of media sales for MSLO, running all sales for digital, magazines and broadcasting, a move that keeps her full-time in NYC. One reason she left: sources say Kelley was tiring of all the travel back and forth to Sunnyvale with all the Yahoo reorgs.]

Sources at the company said Kelley is not the only one taking off in the upcoming few weeks (and last week brought the departure of marketing head Cammie Dunaway to Nintendo and also longtime PR exec Joanna Stevens).

Here’s the late Friday memo from Yahoo Executive Vice President Hilary Schneider on the Kelley departure:

From: Hilary Schneider
Sent: Friday, October 26, 2007 8:33 PM
To: [XXX]
Subject: Sales Announcement

It is with mixed emotions that I am announcing that today is Jacki Kelley’s last day at Yahoo!. Since joining Yahoo! in May 2006, Jacki has led our advertising efforts for the travel category and has recently played an integral role helping to facilitate the integration of Yahoo!’s search and display sales teams. Please join me in thanking Jacki for her contributions to Yahoo!, and in wishing her the best of luck in the next chapter of her career.

To ensure a smooth transition, Jacki’s directs will report to [Dave Karnstedt] on an interim basis. The Advertising Strategy teams have done some great work, and our commitment to their efforts will continue unabated.

We’re making great progress on integrating the Sales Team, as well as with work we’re doing to ensure greater focus across the entire Global Partner Solutions division. The work we are doing now to align around customer needs will accelerate our ability to deliver the most value for our advertiser and publisher partners.

Thanks in advance for your continued focus.

Hilary