I’m sorry to have to tell you this but you really do need to see a veterinarian immediately. They don’t tend to show signs of illness so if you see one it’s probably pretty serious. That being said, i understand when money is tight. At the very least I recommend separating them so the other one won’t catch anything. Go back to a basic diet with Just food you know she tolerates with no issues to try and stop the diarrhea. If you are feeding high water foods like watermelon you can try removing those from the diet for more solid fruits instead. Be careful with this because removing excess water is okay (my boy gets diarrhea with too much watermelon) but removing necessary water while she has diarrhea can lead to dehydration. Make sure she has access to water at all times.
Aggression can definitely be a sign of illness because she’s trying to act like she’s healthy so the other one doesn’t sense her weakness. The best thing you can do right now is go to a vet :
What is her diet right now?