Skin Problems Just noticed a white scabby looking spot on my guinea pigs head

That happened in just a day? Where did you buy them from? How long have you had them for? How did you introduce them? She needs to see a vet, no exceptions. The longer you wait, the more it'll get infected. You need to take her to an EXOTICS vet immediately. Separate them now. When you say that the other one is aggressive, what behaviors does she display? Does she bully the injured one? Does she bite her or lunge at her?

It looks like a fungal infection. You can get an antifungal cream from a pharmacy and treat at home, you just need to be careful to rub the cream in well so that she can't rub any off and eat it. Have a read of this page, which includes information about exactly which kinds of creams to use https://www.guinealynx.info/fungus.html

Your other guinea pig has already been thoroughly exposed to the fungus, so the illness is not a reason to separate them but do keep a close watch on them because of the aggression. Also keep an eye on your other guinea pig, if they also develop fungal lesions then the treatment is the same unless the lesion is right next to the eye in which case you would need to go to a vet to get something that is safe to use around the eye.

I took her back into the store we got her from and they said they would pay for the vet bill so we are taking her in. We've had her for 2 weeks. They were together when we bought them so no introduction was needed. The healthy one has been nipping and lunging at Scarlet. I dont think she has actually bit her though.

Did you get them from a petstore? Nipping and lunging does not call for them to be separated if there's no blood. Only separate if blood is drawn. Did the place you got them from offer their own vet, or are they paying for the vet you choose? Make sure that the vet is experienced and that it's an exotics vet.

I agree with @Soecara. I'd go get an antifungal cream and try that for several days to see if it improves. If it gets worse, or hasn't improved in several days time, you could try an antibiotic cream. In either case, apply it sparingly twice a day with a q-tip and rub it in well.

If you bought a pet-store cage, it's not large enough for two pigs. See the main page of this site for cage size recommendations, and remember those are minimums. More space is better.

Here's some recommended reading for new pig owners:

https://www.guineapigcages.com/forum/threads/113176-Information-for-new-guinea-pig-owners
https://www.guineapigcages.com/forum/threads/107827-What-NOT-to-do-for-your-guinea-pigs!