The Epitaph is a sub-stand, apart of King Crimson itself (on its forehead). Both Doppio and Diavolo used KC and the Epitaph, and Diavolo had given access to Doppio to use the Epitaph and KC's fist.
No Epitaph is Doppios stand that's why Doppio can use it so easly King Crimson is Diavolo that's why Doppio can only use it when he's half and half with Mr.Bossu