Plural popes ... if you have two of them, which is the genuine pontiff?

Today (although the Mass is Of The Sunday) is the festival of S Silverius, Pope and Martyr. He and his historical period raise interesting questions with possible applications to our own equally troubled times.

QUAERITUR:

Suppose you have an acknowledged pope ... but some usurper, by whatever means, pushes him aside and takes his place ... who is then the real pope?

The early history of the Roman See rather suggests that the principle of de facto applies in most situations. The chap who's actually in control ... in Canon Arthur Couratin's phrase, has his bum on the seat ... possesses, as we English say, nine tenths of the law.

But, just for the sake of the argument, let's suppose a more de iure sort of approach. According to this logic, if a usurper usurped without any legality, we would need to say that the displaced pope is 'really' still the 'real' pope. The bloke who displaced would merely be an illegal usurper ... an antipope, in fact.

So far, straightforward enough.

But suppose the displaced but still lawful pope subsequently dies ... what is then the position? (1) Should we say "Right! The See is now vacant, and so the normal procedures for electing a new pope need to take place."? (2) Or should we cheerfully and pragmatically say "Ah well, it was awkward when we had two possible popes, but now the competition has been reduced to one ... let's just live quietly and peaceably under the survivor. No need for Conclaves! Viva il Papa!"? 

Here is a summary answer from a reliable source: 

"After the taking of Rome by Belisarius [Pope St Silverius] was arrested, owing to the intrigues of the ambitious Vigilius, and died in exile at a date which is not known. Vigilius had taken his place on 29 March, 537.

"Having usurped the Papal throne by illegitimate means, Vigilius received universal recognition after the death of Silverius, and thus became lawful Pope." [My emphasis.]

And here is Dom Gueranger with a similar approach:

"The usurper could but be an intruder; until such time as the all-powerful merits of thy glorious death had obtained the transformation of the hireling into the legitimate Pastor, and had made this Vigilius become the heir of thine own courage."

And Gueranger also makes this observation about the questionable succession of S Silverius himself: "The inevitable play of human passions, interfering in the election of the Vicar of Christ, may perchance for a while render uncertain the transmission of spiritual power. But when it is proved that the Church ... acknowledges in the person of a certain pope, until then doubtful, the true Sovereign Pontiff, this her very recognition is a proof that, from that moment at least, the occupant of the Apostolic See is as such invested by God himself."

D'you think these precedents are clear enough?

(In case you were wondering ... the 'deposition' of S Silverius was, according to the Liber Pontificalis, performed by a subdeacon removing the pallium from his neck and dressing him as a monk.)