While you can easily melt a foil pan over a hot campfire, yours will be filled with juicy peaches - so assuming you don't over-cook it, you should be safe. 40 minutes directly on the coals might be pushing it though - consider using a gridiron to control the heat by elevating the pan slightly, and stick around to keep an eye on things... Remember, dutch ovens are thick, heavy, and slow to transmit heat, and the recipes reflect this.
I would avoid doubling either the pan or the foil: you'll end up with an air gap between the two layers, which will likely just end up ruining the outer pan. Pay a little extra for the thicker "heavy-duty" foil and you should be fine.
Wet sand between two pans is an interesting idea... If you try it, report back!
My biggest worry here would be burning the peaches before they have a chance to release their juices (no slow warm-up with aluminum - you'll essentially be frying them). If you can elevate the pan, you should be fine; otherwise, consider macerating them first (if you have very ripe, juicy peaches, count your blessings and ignore this suggestion).
Finally, consider that Harbor Freight sells cast-iron dutch ovens for $25. The quality isn't stellar, and it's extra weight to lug around, but you'll probably end up with better food all the same.
