I've seen some back and forths on this. A lot of explanations that contradict each other. As I've come to know it, there is no such thing as Spell Resistance stacking.
However, I spotted a couple examples where they do. (no I don't remember them off-hand)
Here's an example, though, I wonder about specifically:
PC has racial SR of 25 at lvl 20
PC buys Ring of Splendid Security which has 22 SR of its own.
*gameshow audience* DO! THEY! STAAAAACK!
Thought as much. XD Thank ye.