This is an exercise in ridiculousness, but I find it quite fun to consider - hence powergaming.

Let's consider three characters: a gnomish cavalier (small) with his wolf mount (medium), half-orc druid (medium) and druid's companion, a gorilla (large).

I remember the rules saying that the rider can mount a creature one size category larger than himself. Thus, our druid is able to mount his gorilla (PFSRD even lists gorillas under Ride skill), but not the wolf. But under the same premise nothing stops the gnome from mounting a half-orc, provided the orc is willing. This has tremendous advantages as we can assume the half-orc is combat trained and obviously has more than a few combat uses. We can imagine a gnome piggybacking on a charging half-orc and it seems that in optimising Cavalier this notion is fairly accepted.

However, I am wondering, since half-orc can mount a gorilla and gnome can mount the half-orc, can the gnome mount the half-orc while he's mounted on his gorilla? Is there anything in the rules that prevent this kind of cheese, apart from Rule Zero?