How's this? Take 2 identical riders, same shape, body mass, power output, efficient cadence etc. Put them on identical single bikes, and presumably they have the same W/kg and the same VAM? Now put them on a tandem with exactly double the bike mass of the single bikes - don't know if this is possible? - and ignoring aerodynamic drag differences due to their inline position, essentially pace lining, then presumably their power output is cumulative (and ignoring drive train efficiency differences) so they put twice the power on the road, so their W/kg is 2W/2kg = W/kg ie the same power to weight at they have separately. Hence the same VAM? Presumably once you have differences between rider in mass and power, you get a mass weighted power:weight ratio, which leads to lower VAMs. Eg rider 1 weighs 100kg and generates 200W (2W/kg), and rider 2 is 80 kg and generates 240 W (3 W/kg), then the pairing can ideally generate 440W with a combined mass of 180kg (2.44W/kg). A lower W/kg so a lower VAM. Throw in the effect of the tandem weight increase, above or below double a single bike mass eg a touring vs a racing tandem, and power:weight goes further south?
