There is a difference between efficiency, when one boxing, and playing multiples of the same class/spec.

Still, single play gives us an idea of the potential of multiples.

Assuming abilities overlap, you can also run both Prot heavy and Ret heavy builds, you'd just have to ensure your set up takes into consideration the different special abilities and such.

With 2/2 "Guarded by the Light", your Divine Plea cannot be dispelled.
Not sure on the ability for a Seal to be purged or not.