Don't have access to the game atm. Can you test the following for me please:

Equip a one-handed sword (right-hand) and a spell. Hotkey a torch. Double-tap the torch hotkey. Does the left hand become empty, so you can block with the sword?

Out of curiosity, why does it matter?

Because a spellsword has no way to remove a spell from the left hand short of going into the favorites menu and right-clicking on the equipped spell. Having to do that every time you want to block is too much of a PITA.