What if you've got an unlock spell in the other hand? You have to cast unlock on the enemy after every swing?
No, they have to have a button for each hand and a separate block button.
I don't know, but I'm pretty sure I read that somewhere...
Anyway, we don't need a separate block button.
Wield a shield on one hand: hold button blocks. for example. Or just click, and a double click does a shield bash.
Wield two or only one weapons? Hold the two mouse buttons to block (character crosses his blades in front of him or whatever).
If it's a "one button for each hand gameplay", I think the function will just depend of the object your wielding.