I've noticed that sometimes it says you "dodge" the attack, and other times it says that you "parry" or "deflect" the attack. If these correspond to a failed roll against db and a failed roll against pb, respectively, then I would think you might be able to exploit these messages as a way to test this?
It seems like you could first verify whether or not these messages do indeed correspond to which defense stat caused the attack to fail (e.g. by comparing a defender with high db and low pb, and then vice versa).
Then, if the messages do seem to indicate this, you could use that to test whether the two stats seems to be independent (one roll, followed by the other), or interacting in some way.
Just a thought...