Well removing the skin is not hard on the bad door. On the good door however it is a little trickier, or I should say a little more time consuming. Bad door I will grind the edges off of the skin till the skin breaks loose. You will see three layers of still then ;the door skin, the door frame, and the door skin fold over (which is spot welded).
Toss out the bad door skin. Drill out the spot welds to remove the door skin fold over. You will do this with drill bit that you give a zero angle sharpening too. It is practically flat, and lets you remove small amounts of the spot welds as flat a possible. This makes it possible to drill through the fold over with out entering the door frame.
You will then go to the good panel and using the zero angle bit, drill all of the spot welds. Once that is done, carefully, and slightly unbend the panel bend over to loosen the skin. You will then need to choose one side to unbend even more to allow you to slide the skin off.
You will slide the good skin on the frame you want to use, bend the bend overs back into place. Then fill your spot weld holes with weld. I use my mig for this.
It is harder than I explain, but works great when you have no other options and lots of time and patience.