Hi and welcome
Lymphadenitis is an infection that has usually been contracted elsewhere in the body but that has spread to one or more lymph nodes where it can on occasion cause an abscess. In guinea pigs, lymphadenitis typically affects the lymph nodes in the neck and is most commonly caused by streptococcus. It can appear either in the form of an abscess inside the lypmph nodeor as a swelling of the node.
At this young age, your baby is desperate for company and very dependent on the comfort and guidance of any elder, so any separation should happen only if the companion's life is at a major risk because a separation is a major additional stress factor that will lower the immune response further, especially at an age where the immune system is still under construction. Please be aware that exposure/infection is likely to have already happened, especially if you have bought your piggies together. It takes about 1-2 weeks between infection and outbreak.
What medication has your vet prescribed? And how long have you had your piggies? Is the companion the same age or older?
Here is more information on lymphadenitis. It is thankfully rare in well kept guinea pigs but since yours sounds like they have not been long with you in view of the very young age, they may have rather come with it:
Cervical Lymphadenitis (CL) in the guinea pig - Guinea Lynx Forums