Normally, you can't end your turn in the same square as an enemy, and if you do somehow manage to be forced into one, you both take large penalties.
But then, there's a feat available to Ratfolk called Underfoot:
You are able to share a space with a hostile creature at least one size category larger than you. You gain a +2 shield bonus to your AC while occupying an opponent’s space, as well as a +2 bonus on all attack rolls against the creature whose space you share. Creatures you share a square with take a –2 penalty on all concentration checks. Entering a space occupied by an opponent provokes an attack of opportunity. This ability does not stack with similar abilities providing a creature benefits for being in another creature’s space, such as a mouser swashbuckler’s underfoot attack deed.
I assume a Ratfolk using Underfoot wouldn't take the normal penalties (Correct me if I'm wrong), but how does attacking work if two creatures are in the same square? Does the enemy take the normal squeezing penalties? Can both of them attack normally (with or without penalties/bonuses)? Does the non-Ratfolk have to step out of the square to attack?
Ah, thank you, the only other forum posts I saw said it couldn't happen, and anything about if it did happen said that you'd use the penalties that allies got when sharing a space.
You either need to be tiny or smaller or have a special ability to enter an opponents square. There's no squeezing either way. Someone in another thread said that was the rule, but if you look in the thread it is very much not the rule.
Someone had a reasonable ad hoc decision about the rules gray area about what to do when you legally moved into a square and then that square became illegal because the unconcious person you were standing on woke up, that they then unreasonably extrapolated to an illegal situation.