Several Silicon Valley headhunters say they're now hesitant to recruit from two of the tech sector's most successful companies—Oracle (ORCL) and EMC (EMC)—because of their testosterone-driven, take-no-prisoners cultures. Managers who thrive there, they say, are often bad fits anywhere else. One recruiter describes Oracle as "Silicon Valley's version of the Bear Stearns trading desk. You've got a company full of men who would all walk over their mothers to get to the top. And EMC isn't too far behind." Oracle declined to comment. But Jack Mollen, executive vice-president for human resources …