I am sorry to hear that Poppy & Suzy aren't getting along.
Sometimes female gerbils, especially siblings in a large enough environment with plenty of resources (including hiding spots, things to play with and chew on and plenty of food), get on well enough that they can live together in the same cage.
But when we have one that is much more aggressive and dominant than the other we can see bullying to the point that blood is drawn, and in those cases we need to separate them. I know it seems sad to live alone, but I suspect the dominant one would prefer to have everything to herself, and the submissive one would prefer not to be bullied. You can put their cages near one another so that they can see and hear each other for some company, but that way no one gets hurt.
If the one that was bitten will let you gently flush her wound with a very dilute mix of warm water and antibacterial liquid had soap, then flush again with sterile saline. If you do this daily, hopefully you can keep the wound clean enough to allow healing without a secondary bacterial infection occurring.
If the wound becomes very red and swollen, there is a foul odor, or a green or yellow discharge from the wound then that can indicate there is a secondary bacterial infection and she should be seen by a veterinarian for an examination and antibiotics. If you see evidence of an infection have her seen as quickly as possible as there is danger of her losing her tail with an infection.
Please let me know if you have any further questions.