[/edit]PH 224 said:Magic Item Categories
You can benefit from only one magic item that you wear in your arms slot even if, practically speaking, you can wear bracers and carry a shield at the same time. You benefit from the item you put on first; any other item you put on in the same item slot doesn't function for you until you take off the first item.
Also note that no magic shields, or arm slot items for that matter, have a permanent AC boosting property. They saved that for Armor. So +2 for a Heavy Shield, magic or no, is the best you can get.
Although you can take the Shield Specialization feat to increase the bonus.