Hijack would be destroyed then replayed, so you get to choose a new target.
If you Hijack a Hijack, the Hijack that got Hijack'd would still be Hijack-ing a card despite a second Hijack Hijack-ing the Hijack played first, before it was Hijack'd by a second Hijack. ... Hijack.
The target card will be given the passive 'Hijacked.' Although, thinking again, there needs to be something done for permanents, so maybe it will just be a
icon. or the word Hijacked would appear right above the card. As opposed to ability names, which appear below it.
Pandemonium would affect how the field is arranged. The Hijacked card is not owned by both Players, it is only controlled by both Players.
Voodoo doll's passive will be dealt to the opponent of the owner.