Gull wing door mods were done fairly routinely amongst the show van people back in the 70s and 80s. If you know any old vanners that have been into bodywork for that long they could probably assist with how they did it.
You'll need some fairly substantial struts to hold up the weight of the door. And yes, Lee was right, the one piece doors lend themselves to it more than the 2 doors do although some of the guys joined the two doors and/or fabricated a new skin for them.
The one problem IIRC was getting the lift high enough that tallish people didn't bonk themselves on the head going in and out. Most of them had the door so it only lifted to the height of the opening.
Oh and word of warning, they won't really provide shelter because there will be a gap between the body and the door when open that will let in the rain.
I too would like a more sheltered back door when opened so rather than installing a 'window' style RV awning at the back I'm sewing up a silnylon cover that will fasten inside the van and come out over the two back doors. It will stop the rain from coming down off the top and into the back six inches of the van floor, a design deficiency that drives me nuts. It will also provide me with a shade awning to keep hot sun from blaring into the inside of the van.