In the results section from http://www.kreuzotter.de/english/espeed.htm#pv , he calculates an effective frontal area. For three same height (67.7 inches) riders two on a tandem and one on a single bike the effective area in the drops for each one is 4.28 and 3.26 for a tandem and single, respectively or a 31% increase in effective area for the tandem. YMMV