I don't know the exact values, but my best rule of thumb is that having two shields is only viable if you are on horse, use a light shield then when you get dehorsed you drop it and take the heavy shield on your back, just like Phazh used to do (I don't know if he still plays like that). Having two shields on foot is more or less suicide, at least in battle mode.
Ssh, spreading my secrets around like it's common knowledge. Thankfully you didn't blab about my ninja 'sheathe sword - drop shield - unsheathe sword - select secondary shield from back' trick when my horse goes down.
Oh drat.
Never use a second shield on back on foot. It slows you down too much. But on a horse, it's viable, although a bit useless. I like it and i'll keep doing it, possibly because the 'sheath sword - drop shield' is a reflex i do every time my horse dies, even when i don't have the second shield on me...