maybe its the same with attack and block but you attack with both/ faster at the same time as you would be just pressing left mouse. so pretty much a normal attack would be a x slash or something along those lines.
They couldn't get away with that because you can wield a weapon and magic at the same time.
I'm thinking they will have two 'hand" buttons, one for each hand and holding down a third button will change the function of each of the "hand" buttons.
For instance there will be two "hand" buttons that sheath and unsheath by default.
Holding down an attack button will change the function of the "hand" buttons to attack.
Holding down a block button will change the function of the "hand" buttons to block.