Hmm, could be early-stage cheilitis. And, yeh, can be bacterial and/or fungal/yeast. OR it can be caused by acidic foods, but usually only one piggy tends to be susceptible...unless you have two that are susceptible!!
To treat yeast/candida, I use Nystatin/Nilstat/Mycostatin which is a human oral treatment available at chemists.
To treat bacterial, sometimes a course of baytril may be needed, but usually only topical use of an antibiotic cream works wonders. If Jess were to come online, she would probably suggest colloidal silver!
The cut looks like a minor attack wound! Have the boys been having minor tiffs?