It doesn't always follow that both pigs would have it as they can get them if they are run down, all pigs have them in their system and they can overpopulate sometimes, so one pig may show symptoms while another doesn't.
I would get a vet appointment and see what they say, if the vet does think it is mites, I would ask for injection rather than Spot on as it is more effective and less likely to cause a reaction to a pig that is already sensitive. Alternatively, Panomec (the injectible form of Ivermectin) can be given orally at a dose of 0.1ml and this is what is given at the piggy clinic I recently volunteered at.
If you think she is in pain at all, they can have children's Nurofen at a dose of 0.2ml twice a day. This is taken from page 493 of the Exotic Animal Formulary by James W Carpenter. This is if you don't have access to prescription pain relief from the vet and she appears to be in pain.