"One of the pilots might have suffered the illusion but surely not both?"
The somatogravic illusion is a mechanically induced illusion for which the maths can determine the magnitude of false pitch. An acceleration of 30 knots over ten seconds will give a false pitch of 9 degrees. If both pilots succomb, they will have the same magnitude of sensation. It is caused by the utricle and saccule of the inner ear sending sending acceleration information to the brain which misinterprets it as pitch in the absence of visual confirmation. As in these cases, visual information tends to be the AI/PFD, this information is not as compelling as the real horizon and can be ignored by the pilot. Anyone who has experienced this illusion will tell you that it is incredibly powerful. It takes all your willpower to supress it and believe your instruments.