This subject reminded me of a paragraph from Chapter 4 of Phil Irving's excellent book Motorcycle Engineering. "While the weight of a thin-walled tube increases almost in direct proportion to its diameter, its torsional strength increases as the cube of the diameter and its deflection (for any given twisting moment applied to it) decreases as the fourth power. Therefore, a single 2 in. tube weighs the same as two 1 in. tubes, but is four times as strong and will twist through only one-eighth the angle for the same load. Put in another way, a single 1 1/4 in. tube is almost as strong as two 1 in. tubes, deflects slightly less for the same twisting moment, but is only five-eights the weight." Colnago Bi-Titans are another example of this recurring fashion. I seem to remember hearing that they're prone to breakage, as well.
Best Regards,
Wes Gadd