2 of my cats were very susceptible to ringworm & it kept coming back, while one cat had it a little bit, and the 4th cat never had any symptoms.
It could be that Jordan's immune system isn't as good. It's also possible that Levi could be an asymptomatic carrier - have you had a culture done of his fur? That would let you know if he has it & it just doesn't show, or if he's clean.
For the immune system booster, I've seen people on this site write about giving Grapefruit Seed Extract to cats, but I've never tried it. If you want to try it, check with your vet to make sure it's OK to take with the medicine you're giving, and to find out the best dosage - I think it only takes a drop or so mixed in water.
I would strongly recommend asking your doctor about trying Program - it worked well for my cats, and is much less expensive than the Griseofulvicin (if that's what you're using). I tried the fulvicin on my cats, but it made 2 of them very sick, so we stopped it. Program worked better, without the side effects.http://www.vetinfo.com/cringwrm.html