Compound Hulk
When Xemnu the Titan attacked Earth to take revenge against the Hulk, both the Green Hulk and Red Hulk were helpless against his army and mental powers. It was only when the two Hulks were combined into one body (thanks to the Impossible Man) that they found the strength to give Xemnu an old-fashioned beatdown.
Go read Hulk #30 by Jeff Parker for the full story behind the Compound Hulk. My short description can't give it justice.
