Having this happen, I switch to one hand to grip both of your wrists tightly above your head, using my other hand to spider tickle your right armpit cooing tickle tickle tickleeeee!!*
*lays back down on the couch, stretching out* Usually I don't have the chance to test if they're lying or not though. I just respond with "Oh, you're lucky. I'm crazy ticklish." And then they're on me lol