So the Ratfolk have a nice ability called 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?