I wouldn't recommend it. Tandem and triplet tubing is much bigger than single-bike tubing. Before the likes of Bill McReady (sp?) at Santana came along and made the tandem world what it is today, one problem with early tandems was that they were much too flexy. Our steel tandem's tubes are the size of Cannondale single-bike aluminum tubes, and the bottom tube is as wide as the BBs.
Anyone with the math skills tol calculate the frame stresses would agree that a tandem or triple bike is a new and different bike from the ground up.