The fact of the matter is that they will do better together, not separated, unless the stroke caused an unusual behavioral problem in your other rat.
As for the amount she drinks and pees... well, I drink a lot more and pee a lot more than my mother in law, LOL. As with people, rats might have different hydration and elimination needs. If she's always been like this, there may not be an issue. If there's been a sudden jump in the amount of drinking and peeing, that probably means there's a problem. Either way, if you think there's a problem, then it might be time for a vet visit.