So the Ratfolk have a nice ability called Swarming:
Swarming
Ratfolk are used to living and fighting communally, and are adept at swarming foes for their own gain and their foes' detriment. Up to two ratfolk can share the same square at the same time. If two ratfolk in the same square attack the same foe, they are considered to be flanking that foe as if they were in two opposite squares.
So my question is can both of them ride the same mount by RAW? If so, as far as I am aware, your mount has to be one size larger than you, because that is how much space is available for riding. Since they take up the space of one creature, they should be able to ride a medium sized mount?
Best Answer
In my first completed campaign, which was Indonesia themed, one the the players was a rat folk with leadership. He had heaps of followers and would ride an Indian elephant. We figured out how many rat folk could ride on it by doubling how the number of medium people that could ride it. Anyone riding the elephant had to have riding related feats.
I never had any issues with running the game that way, since half of his followers kept falling off the damn thing, and the other half could barely hold a weapon.
Long story short, 2 rat folk riding a horse at the same time should not be an issue, assuming they both have the appropriate riding feats. The core rule book and all the other books I own at the moment are very vague and unhelpful on the topic, so I can't quote anything.
Someone else already mentioned it, but a custom/exotic saddle will definitely be required.