Lets say the pair is a Level 5 Monk (Vanilla) and a Level 5 Wizard. The Wizard casts Enlarge Person and wants to basically ride on the shoulders of the monk. It seems reasonable at first for a movement means for the Wizard as the monk has Evasion, Slowfall, Fast Movement, and High Jump by the time he reaches 5th.
Is it doable? Does the wizard recieve cover from the large monk he is riding? Does the Monk need to walk around with an exotic saddle just so the Wizard can sit on the Large Monk's shoulders?
I am fairly certain that you cannot ride bipeds at all.
edit: the monk could possibly carry the wizard though, but that follows different rules.
Absalom's Axe Beak riders beg to differ.
And a thorough search of the PRD comes up empty as well. Probably leftovers from older games in my head.
edit: the only thing that DOES come up is that biped eidolons cannot take the mount evolution (but eidolons often come with their special rules)
Also, the cavalier is restricted to 4 legged creatures for a mount (although the beast rider has the same restriction... and can select a t-rex).
It doesn't specify that in the cavalier's description, though. All creatures on the standard list just happen to be four-legged.
Sorry, it was in the beast rider description. The actual cavalier mount just has an exhaustive list.
A beast rider cannot choose a mount that is not capable of bearing his weight, that has fewer than four legs, or that has a fly speed (although the GM may allow mounts with a swim speed in certain environments).
First, ask yourself this question: Does the idea of a wizard riding a monk around strike me as silly and ridiculous, or is it awesome and cool? To me, the obvious answer is that its ridiculous and not what I particularly want in my heroic fantasy, but maybe that's just me. If you think its cool and its making your game more fun, maybe its ok to let your players turn the combat into a chase scene from Benny Hill. If its causing problems, then you should do everything in your power to hinder them from breaking the game without seeming unfair.
I dealt with a similar issue as this in 3.5 with a halfling cleric and a human barbarian. The halfling enjoyed riding "piggyback" on the barbarian's shoulders. At first I saw no problem with this, and I didn't even provide any kind of detriment to the barbarian's movement or penalties of any kind for having the halfling on his shoulders. Eventually some clear issues arose, however.
When riding a mount, generally you and your mount will share initiative and act on the same turn, so that the mount's movement counts as your own move action. I enforce this in my games now with no exceptions. What can happen if you allow them to act on separate initiatives is that the character riding the mount could be moved on the mount's turn while it moves, then on their own turn the rider could hop off and move again, thus moving much farther in one round than should be possible. The halfling cleric in my game was getting the best of both worlds before I put a stop to it as riding the barbarian around was giving her far greater mobility, and the barbarian got to have the healer on his shoulders most of the time. The barbarian would charge into combat, then on the cleric's turn she would sometimes heal him, hop off as a free action with a ride check, then move toward another ally who needed healing. If I were you, I would be sure to stop any such nonsense from occurring. As DMs, we must be ever vigilant against tomfoolery.
Also, keep in mind that riding a creature that is ill suited as a mount causes a -5 penalty to ride checks, as does riding without a saddle. If you are going to let your players ride each other around, at least be sure to enforce such rules as these. Should the wizard take a hit while riding the monk, then he needs to make a DC 5 ride check to stay on, and with that -10 penalty that might not be so easy. As for gaining cover from the monk, the wizard has to make a DC 15 ride check to do so, and again with those penalties in mind this might be impossible depending on how good a ride check the wizard has. And remember that while taking cover behind a mount a character cannot attack or cast spells until they leave that position with a move action. You should also be sure to consider encumbrance penalties caused to the monk for packing the wizard (and all his gear, coins, etc) around. Normally I stay as far away from encumbrance as possible, but sometimes you have to do what it takes to keep this kind of silliness out of your game.
Meanwhile, I would also think that having a person on your shoulders, weight aside, should cause some kind of penalty simply due to awkwardness. I know of no official rules governing this, but were I you I would be unafraid to invent some.