One tie you could give a go is tying them standing, face to face with their hands tied above their heads and then back down to the other's tightly tied crotch rope. Then take to sets of clothes pins and make two sets of pins taped back to back. Attach these to the girls across from each other and then let the tickling/ flogging/ vibrating ensue. As one moves she will pull on the other's rope as well as give all of the nips a pull. Just a thought,
Also, you could always just take them out on campus with one's right hand simply tied to the other's left and let them go for a stroll.
-cj