Board Thread:Questions and Answers/@comment-28702329-20180609141525/@comment-34882417-20190209092544

Sticky Fingers Ari wrote: The fact that DIO subconsciously knew that someone was watching him in Stardust Crusaders after the Hermit Purple Picture was taken. If you were to say that it was because of Jonathan's Late Stand, then it shouldn't be possible due to DIO having started the Stands in the Joestar Bloodline. Also, one person can't have two stands (Just different forms or abilites like with Echoes and Killer Queen). Yeah, DIO started the Stands in the Joestar bloodline...

After being stabbed with a Stand Arrow. While he had Jonathan's torso, arms and lower body. Thus awakening Jonathan's Stand. It's not that complicated.

Also no, it's been shown that a Stand user can use the Stands of deceased individuals as long as they possess their bodies. Diavolo was still able to use Epitaph ALONGSIDE King Crimson, even after Doppio was forced out of his body and then promptly killed. It stands to reason that the same should apply to DIO and Jonathan.