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?
I wanted to have a set of characters ranging from as small as I can get (currently with a Small race - any Tiny playable characters out there?) up to having a Huge mount (say, a Mammoth with a Mammoth Rider PRC) giving me an effective stack of 4 (Small, Medium, Large, Huge) characters, though not necessarily PCs, say, animal companions, summons, familiars, just regular mounts. After gathering a number of creatures willing to participate, I'd make sure the top one is a Barbarian with Ferocious Mount and Greater Ferocious Mount. This causes the barbarian's mount to use his rage and all of his rage powers that do not require an action to activate and are constant when raging. It just so happens that Ferocious Mount and Greater Ferocious Mount are such powers, so in my understanding all the applicable rage powers would propagate down the Stack'O'Mounts fueling this ungodly tower. Now the focus would be to enhance each mount's attacks, be those natural attacks or otherwise. However, if Stack'O'Mounts could possibly charge, Beast Totem theme has some very powerful choices. If not, then maybe getting a wereboar-kin to be on top could enhance each mount's gore attacks, that could be a lot of Con damage.