I'm a complete BWT (Big wall Theorist), so can you answer a couple questions:
Ahemm. Have any of the posters ever done a piggy rap?
One rope?? OMG. Shut the fuck up will you. If you are rapping the route, or just the normal descent, it makes no sense to complicate anything and just go with the normal procedures.. Rapping with a pig is no big deal, just ride the sucker till you discover your oops.
Tie a backup to the piggies rap device, (You were riding the pig, right?) Get you ascenders off your harness and ascend to the anchors. Anchor yourself and set up your haul. Anchor your piggie, and get back on board.
Pull ropes from last station, get back on rappel, and rinse and repeat till you are on the deck.
Why in the world would someone have a sixty meter rap, using two ropes, and only use one and a grigri?
Big wall theory? Just go with standard operating procedures. If there is a shortcut, please oh please let it come from someone with some wall mileage, not from theory on the intardnet.