Weight of tamdem versus two solo bikes

A question posed to the New Scientist's 4 July issue posits
that multi-seat bikes have a total weight advantage over
solo bikes (per seat), asking at what seat count the weight
advantage per seat fades away.

Initially, I thought the premise of the question mistaken,
but now I'm curious: Is it true that a modern tandem weighs
less than two solo bikes of the same total carrying capacity?

The illustration that accompanied the question is of a triple,
which I've never seen anywhere in the wild and was probably
chosen for its attention getting value. It's on page 56, alas
no weblink is offered.

Thanks for reading,

bob prohaska