Only the highest enhancement bonus applies
The Dragon #318 article "Oriental Adventures Update: Eastern Flavor" on Magic Chahar-ainas and Dastanas says
Both dastanas and chahar-ainas provide special armor bonuses to AC that stack with other armor bonuses granted by certain forms of light armor. However, it is still the case that only one enhancement bonus can apply to a character’s armor bonus at any time. This, if a character wears +2 dastanas, a +1 chahar-aina, and +1 cloth armor, only the +2 bonus fom the dastanas increases his AC. A character can still gain the benefit of special abilities attached to multiple pieces of armor, however, so a character wearing +2 balance dastanas and a +2 displacement chahar-aina has a +2 enhancement bonus to AC and can use the balance and displacement abilities. (42)
Note that the Arms and Equipment Guide represents the dastanas (15), superseding Oriental Adventures, but changes little except putting the dastana into the Shield category exclusively (rather than OA's Shields and Other Additions category). The chahar-aina remains exclusive to Oriental Adventures.