It seems that there is one button assigned to each hand. In the case of the shield, the action being a block or a shield bash, is decided by the time you press the button. Apparently the inability to block affects not only dual wield, but one-handed weapons as a whole.
The (apparent) lack of a system, similar to the one used for shields, for one-handed weapons, could be because power attacks will be used again.
IF I am correct, as an idea for future TES tittles (as I suppose it's to late for this one), or for mods, I think, the same way pressing the shield button some time change the block into a bash attack, power attacks could include a block in the beginning of the movement, as waiting in a menacing pose before attacking, didn't make much sense anyway.