Yeah, the way the rule reads to me is that the secondary player would need to change his helmet. On top of that, he can't just drop the green dot helmet off in the bench area; he has to return it to the communications area behind the bench and hand it back to the NFL official, who puts it back in the trunk and logs it back in like a librarian checking bokks in and out.
In other words, it's going to be used by one player and one player only - unless the primary player leaves the game due to injury.
patchick - I agree, either Vrabel or AD since they would be the players most likely to be on the field for the largest number of plays.