But, that's a bad example because weapon % effects are automatic, are they not? So, unless someone has outfitted the animal with a mouthpiece of Tendonslice that happens to process with their Improved Trip, they should not be getting two effects in one hit. These are animals, not baddies with weapons. And even if it was a baddie with a weapon, the effect would be % to process and automatic, not save based. Right? (based on weighted as an automatic stun with no save, just % chance to process.)
It makes more sense to me to have a Lion get Hamstring with it's bite attack and Trip with it's claw attack (or vice versa). Two separate attack rolls, two separate combat feats within the same attack chain. Oh, and on a real 10sec timer so it can't do the same thing 3 seconds later in it's next attack chain (1 out of 3 combos with combat feats would be about right).
P.S. When will players get Improved Hamstring like the Rogues in the Orchard? I want to be able to Trip and Hamstring at the same time!