Oddly enough, I've found thongs to be the best undergarments to keep the twins dry. As an added bonus, they prevent swamp-ass by giving the sweat a way to escape.

Gotta go with cotton or wool, though. Synthetic fibers just don't cut it. I hate recommending name brand products, but after a ton of wasted money on Chinese brands on Amazon, Jockey thongs have performed the best. There's also a Czech woman on Etsy who makes them out of Merino wool, but they're like $30 a pair.

Source: I have hyperhidrosis (I pour sweat for no reason despite being in good shape), and sweat induced psoriasis so keeping my scrotum dry is of the utmost importance.