This comes from the book Seeker of Secrets.
The Dusty Rose Prism Ioun stone provides the user with a +1 insight bonus on AC. When slotted into a Wayfinder, Ioun stones provide another bonus in addition to their normal bonus. For the Dusty Rose Prism, this additional bonus is a +2 insight bonus on CMB and CMD.
I'm not completely sure whether the combined effect of this is +2 or +3 to CMD. Both the +1 and +2 are insight bonuses, but they are not insight bonuses to the same thing, even though they both get applied to CMD.
At first I thought that they would not stack, and only the highest would apply as they're both insight bonuses. However, I think I've changed my mind after thinking about how enhancement bonuses work.
Amulet of Natural AC: +1 enhancement bonus to natural armor.
+1 Full Plate: +9 armor AC, +1 enhancement bonus to armor AC
+1 light wooden shield: +1 shield AC, +1 enhancement bonus to shield AC.
Each of these three enhancement bonuses apply to the character's AC, because they are not strictly speaking enhancement bonuses to the same thing (though there are statistics that all of them apply to; normal AC and flat-footed AC).
Likewise, while a +1 insight on AC and a +2 insight on CMD both are insight bonuses which apply to CMD, they are not strictly speaking insight bonuses to the same raw statistic, so do they both apply when determining total CMD? I'm still unsure due to the fact that while the enhancement bonuses apply indirectly (AC does not directly include the enhancement bonuses, but rather the Nat, Armor, and Shield bonuses they enhance) while the insight bonuses both apply directly to CMD.
Thoughts?
As far as I can tell, from RAW, it's just the +2 to CMD. The reason for this is that you don't actually apply AC bonuses to CMD, but rather most things that apply to AC also apply to CMD. In this case, the +1 insight bonus from the regular ioun stone results in +1 AC and +1 CMD. Slotting it makes the bonuses +1 AC and +2 CMD.
Your reasoning with the enhancement bonuses is faulty. Natural Armor, Armor, Shield, Dexterity, Deflection, Dodge, etc. are all bonuses which accumulate to make Armor Class. You can enhance each portion separately before they are added. You are modifying individual things which create an aggregate score. CMD is also an aggregate score, but one that does not directly take AC into effect. You gather many of the same components (Dodge, Deflection, Dexterity, etc.) but one does not wholly include the other. Oftentimes, an item or ability will call out that it effects both AC and CMD, but this is likely because the fact that many of the same bonuses apply to AC and CMD is hidden in the rules away from where you'd expect it to be.