my silkroad is made from columbus zona oversized asymmetric double-butted tubing. The downtube is ovalized vertically up near the headtube, and horizontally down near the BB... This design is very similar in theory to the tandem boom tubes you mention.

At some point you are making the tubes more dent prone as you thin them out and make them larger diameter... I think OS tubing is the best 'durable' compromise.

OP, the flex isnt a 'problem', rather, it is a feature which is sometimes more or less desirable.