I know that you can only hold one shield at a time but the description of the Aegis of the Raven says that its plus magic bonus to AC stacks up with the current pluses from a shield that is currently used. This description about bonuses does not make sense if you can't wield both shields at the same time.
Here is the description:
While holding this shield, you have a +3 bonus to AC. This bonus is in addition to the shield's normal bonus to AC.