Short version: Can a creature mount a creature that is already mounted?
Long version: "Undersized mount" allows a creature to ride a mount of the same size as the rider. "Gang Up" allows a creature to be considered flanking as long as they are threatened by 3+ creatures with the feat. I'm looking to combine these, and do that thing from the little rascals, having three gnomes stacked on top of each other who all have sneak attack. Please tell me this is a possibility. I know it's possible to do it with just two ratfolk, but that can't be taken to the pathfinder society. Alternate idea: do it with wayang, who have no strength penalty. Maybe dwarf->wayang->halfling.