According to Avrille, the rebuttal to sedevacantism about the claimants since Vatican II is that they are the popes of two different churches/religions.
I am wondering how this does not reduce the position down to the claimant to being Christ's Vicar one day, and Satan's the next? He works to build the Church for Christ one moment, and then carry out the work of devil the next?
Could Francis, for example, be the pope and at the same time the Archbishop of Canterbury (Anglican) according to traditional ecclesiology?