Cylinder heads or gaskets don't need to be replaced in pairs. No need to crack open the other side if there are no issues.

Your post is a bit confusing. You say they said both heads should be replaced or both gaskets? Either way, absolutely not.

If the extended warranty for the defective head applies to your car, there should be no cost to you at all.

 

It is not uncommon to only service one head vs the other. I agree with Dave in that you shouldn't break open the head unless you absolutely need to. Saves a lot of time and money too!