No idea how long they've been sick from this post, so that's impossible to say. When did you get the first snake? And the second?
Has the first one been kept in strict quarantine from the other? If not, the second one may have caught what the first one brought.
I don't think humidity by itself can actually cause an RI*- only facilitate whatever pathogens are lurking, & their immune resistance is reduced from the stress of shipping & being in a scary new home- I'm not saying "your" home is scary, just that ALL new places are scary for snakes, so they can get sick easier.
I'm not familiar with giving antibiotics orally to snakes- they're usually not thought to be effective that way, & therefore are usually injected. Antibiotics are also not harmless- they do have side effects- they're a bit rough on a snake's body (stomach, liver, kidney) & especially 2 at the same time. BTW, while I'm thinking about it- after antibiotic treatments are completed, consider adding reptile probiotics to their prey for a while, to help their stomach. (either Nutribac or Bene-Bac)
Are you giving the injections or is the vet? If it's you, have you asked the vet about the snake being bloated- or puffy with air? You need to ask a.s.a.p.-I'd be very concerned about that.
If you need another vet- one more experienced with herps- may I suggest this site: https://arav.site-ym.com/search/custom.asp?id=3661
In my experience, vets (& other doctors) usually project confidence in their treatment- we'd run the other way if they didn't, so right or wrong, they're usually "confident".I wish it meant that they're always right- but you'll have to be the judge.
There's not much help we can be online, unfortunately- we cannot take the place of real veterinary care. I hope they both recover & soon. Did your vet say what the culture showed? What are they being treated for? What are their expectations?