Because if you only have to use one at a time it can be a devastating combo.
As Drealin points out they would both trigger as you are required to Reveal them at the same time and on the same trigger so both would be used on the same attack.
Also, if they seeking dispel one of them you still have the other to protect your Mage or creature.
This is true but since they both cover the same type of spells and have the same trigger you'd be wasting them once a spell was cast that they protect against. You can't stack protection like that in Mage Wars