I think you need to remove one of them and then tickle each foot one at a time. If the bare foot is more ticklish, they are socks and against the rules, so you'll need to take the other off and tickle both feet. If the bare foot is less ticklish, they're nylons and you should put the other back on and then tickle both feet. Either way, she gets tickled with whatever makes her feet more ticklish, so you win ;-)