If a 1h/2h use a shield they get a penalty. If they are used on horse they get a penalty. They are the same amount BUT they do not stack, so using a 1h/2h with shield on horse is the same as using one with a shield on foot or on a horse. 2h used from horse has the same penalty. This is because you normally have high speed and thus high speed bonus so you will still do more than normal damage.