"We've received many e-mails ? the last few months regarding how to overclock two Radeon HD 2900 XT in CrossFire mode. Unfortunately we haven't had a good answer for you, but with some help from macci, now a resident of AMD/ATI, we can now present this guide, which shows you how to overclock Radeon HD 2900 XT when you have two of them running in CrossFire mode."
Check it out at Nordic Hardware.