UPDATE 2-Du Plessis to chair SABMiller in addition to Rio Tinto
* Du Plessis to join board in Sept, become chairman in July
* Hopes to keep two chairmanships, which some see as difficult
* Emerging market experience seen as a plus
* SABMiller shares down 2 pct; Rio Tinto shares flat (Adds background, comments from du Plessis, analysts)
LONDON, Aug 6 (Reuters) - Jan du Plessis is set to become chairman of two big global companies at the same time when he takes over the role at brewer SABMiller Plc next year in addition to his current position at Rio Tinto.
Du Plessis, 60, will join the brewer's board on Sept. 1 and then succeed John Manser as chairman following its annual shareholder meeting in July 2015, SABMiller said on Wednesday.
Du Plessis said he would like to continue serving as chairman of Australian mining giant Rio Tinto, though he plans to step down from the board of retailer Marks and Spencer Group , where he is currently a director, in the first half of 2015.
Given the size of the two FTSE 100 companies, some critics questioned whether one man could effectively serve both roles.
"We're puzzled why the company would think someone who already has a very complex role as chair of one of the largest diversified resource companies in the world could effectively also chair SAB, and more effectively than anyone else," said Andrew Whiley, spokesman for PIRC, a shareholder advisory firm.
PIRC generally opposes multiple chairmanships, Whiley said.
A spokesman for SABMiller, the world's second-largest brewer, said it was "not unusual for board members to have multiple roles on companies and other outside commitments.
"Jan will spend as much time as is necessary to fulfill his duties as chairman," spokesman Richard Farnsworth said.
SABMiller shares were down 2 percent at 3,151 pence by 1130 GMT, underperforming the FTSE 100, which was down 1.3 percent. Rio Tinto shares were down 0.8 percent.
Du Plessis - who likes James Bond films, Puccini operas and skiing - said the SABMiller appointment "does not in any way diminish my strong commitment to Rio Tinto.
"I have assured the board of Rio Tinto that I would like to serve as chairman for several more years, subject of course to the board and shareholders wishing me to do so," the Capetown native said in a statement.
If Rio Tinto decides later on that du Plessis can not devote adequate time to the job, the company could be in a tough position, Canaccord analyst Peter Mallin-Jones said, citing expectations that Chief Executive Sam Walsh may retire at the end of 2015.
"To change both senior roles around the same time may be a bit difficult," Mallin-Jones said.
Walsh, 64, was appointed CEO in January 2013 for a three-year period. Because of his age, some analysts think he will retire at the end of the term, though he recently told a British newspaper he wants to stay as long as the board wants him to.
For SABMiller, which was also born in South Africa and has dominant positions in a range of emerging markets including Africa and China, du Plessis brings valuable experience.
He has degrees in commerce and law from the University of Stellenbosch and qualified as a chartered accountant in the country.
Du Plessis served as group finance director of Swiss luxury group Richemont from 1988 until 2004. He joined the board of British American Tobacco in 1999 and became its chairman in 2004, a position he held until 2009. He also served as a director of Lloyds Banking Group from 2005 and chairman of its audit committee in 2008-2009.
"In principle, the experience and the background sound good," Bernstein Research analyst Trevor Stirling said.
SABMiller appointed John Manser as chairman in December following the death from cancer of Graham Mackay, who spent 35 years at the company, steering its transformation from a South African industrial conglomerate to a brewer with 200 brands including Miller Lite and Peroni. (Additional reporting by Silvia Antonioli; Editing by Mark Potter and Jane Baird)