The question title sums it up fairly quickly. If a character is wielding two shields, one in each hand, does she gain the shield bonus to AC from both, or only the higher one? And where in RAW is this written?
Two shields do not stack. They both offer a shield bonus to your armor class, and like bonuses only overlap.
You can find the rule stated clearly here: http://www.d20pfsrd.com/equipment---final/armor#TOC-Armor-Shield-Bonus
Shield bonuses do not stack with other effects that grant a shield bonus, but nothing says that the bonuses from two shields do not stack. The rules on Armor/Shield Bonuses says
Each type of armor grants an armor bonus to Armor Class, while shields grant a shield bonus to Armor Class. The armor bonus from a suit of armor doesn't stack with other effects or items that grant an armor bonus. Similarly, the shield bonus from a shield doesn't stack with other effects that grant a shield bonus.
(Emphasis mine.) So while a breastplate doesn't stack with a chain shirt, two shields provide bonuses and aren't themselves effects.