I've read the counter arguments in this thread why this or something similar should not be done. I don't find them persuasive at all.
Firstly - duel wielding does not gimp your damage. In fact - it increases it.
So there goes the argument that there is some kind of damage penalty if you choose not to use a two-hander and therefore the extra set bonus is needed to balance things out.
Secondly - and as someone who uses two-handers (staves) let me just say there is nothing in the Destruction Stave skill line for example that is over-powered when compared to the duel wield skill line. There just isn't.
So both counter arguments fall flat as far as I'm concerned. I remain in the OP's corner - and believe characters who use two-handed weapons should have access to just as many set bonuses as characters who use one-handed weapons.