Among the Grub Street Guide to 41 Hyper-Regional Sandwiches is the Sheboygan Double Brat.
The researchers didn't finish their homework, as they left out the Point Drive In, a Karlson family tradition since the beginning of the Vince Lombardi era. They correctly noted, though, that a Sheboygan style double brat has grilled onions on it. The Cold Spring Shops version has sauerkraut, a heresy the Superintendent acquired in Milwaukee. (They're catching on in Illinois, too.)
If you want to cook your own, here are Johnsonville's instructions, and mine. That roll is an honest-to-City Bakery hard roll. I load up with a supply at the beginning of the summer. The Jewel bolillo rolls will hold up well enough in an emergency. Hamburger buns are too flimsy.