Hi and welcome
Guinea pigs only rarely bite each other, especially sows. There version of a serious bite is a mouth full of hair.
Most wounds are generally self inflicted from the discomfort of ringworm or mange mites. However, we cannot even start to guess what is going on.
If at all possible see a vet. You can reclaim the cost from your pet shop because they have been exposed to whatever is making the rounds there and you have been in effect sold damaged ware. Do not home treat on spec with low dosed pet shop products which can't heal, only prolong the suffering and make the shop more money thanks to their bad practice.
Here is more information:
New guinea pigs: Sexing, vet checks&customer rights, URI, ringworm and parasites
COVID-19 - Vet visits/emergencies during coronavirus/Covid-19 lockdown
Please be aware that guinea pigs don't time share. Every meeting means a renewed bonding process with dominance behaviour before they are separated not even halfway through and looked in an isolation cell. They can never build up a relationship like that. It is also extremely stressful for them, which helps opportunistic illnesses and parasites getting a foot in.
Take the time to learn understanding interactive behaviours during the complex bonding process, which lasts in all about 2 weeks, so you get a much better idea of what is going on. How big is your cage and have your girls space to get away from each other?
PS: Have you made sure that they are actually girls? We see so many sexing mistakes by shop personnel on here.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Sows: Behaviour and female health problems (including ovarian cysts)
Cage Size Guide