Rings copy activated abilities. The bracers' ability is triggered, so it can't be copied with rings. You'd just get two copies and the original. You should also try Thousand-Year Elixir. You could untap krenko twice with rings. For 9 mana, you could tap krenko 3 times and get 6 copies. Starting at 10 would leave you with 10240. Well, even without doubling 9 times, any of those cards make krenko pretty scary.