Actually most of them haven't. For the most part voidsent that are on the Source are there by some form of contract, that means that when they 'die' on the Source they just go back to the Thirteenth. It's a vast minority of voidsent that are on the Source in their true physical form.

There's also no indication that 'soul twins' really... like, matter except in the cases of an actual Rejoining. They exist as a concept, but don't assume weird magic would happen if you put Rowena and Mowen in the same room.