If you're worried about an element touching the wort directly, use a HERM coil. Going with a double-jacketed system for mashing seems like a ton of expense for something that probably doesn't warrant that much complexity.
jmo, of course.
EDIT. Sorry. I misread. You want this for the boil. I still think double-jacketed for heating is overkill.
Muddy Creek Brewery - Opening Fall 2014
Muddy Creek Chocolate Stout: So good we named the brewery after it!
Skinny Cow IPA: In India, cows are sacred, that doesn't mean they feed em...
Good Time Wheat Ale: Here I am, Havin' a Good Time!
No Paddle Amber Ale: Up the creek without a paddle, But Damn the beer is good!
Dirty Blonde Ale: You know you want one.