Something we have done on other long hikes is exchange spare keys with another couple, and set up a shuttle. One group hikes down, the other up. You meet in the middle for a nice lunch together, then complete the hikes and drive the other's vehicle back to camp.
The issue with Marble Canyon in DVNP is the roads, miles, and driving time - far far far from a usual shuttle. First we'll do the upper section down to Deadhorse and back (as Occidental suggests), or possibly ascend a connecting canyon for the back up and connect with some cross country. We will still consider a through hike but the best scenario would be a ride in and dropped off by a very kind soul at Goldbelt, sleep under the stars, and descend Marble the next day to our waiting camper at the bottom.