Basically what Markg said is the way to go. You can vary your cage furnishings, but smaller is usually better with baby dumerils. I keep all of my newborn dumerils in the good ol' .98 cent rubbermaid shoe boxes, like you can buy at any wal-mart(well, until they changed styles again, but anything in that size range will work). Anyway, the 4' cage even split in two is going to be too big for a baby dumerils. You might get an established feeder to do well in that size of cage, but GENERALLY, you'll run into problems such as the one your having now. Also...placing a new animal right next to another recent addition is a bad idea. What if the R.I. isn't completely cleared up and you end up with two sick snakes. OR...you bring in a new snake which is just waiting for the right stress factor to bring on an illness and it gets your recently treated animal sick AGAIN....MORE HIGH VET BILLS-always best to avoid those-LOL! My baby set up is simple....plastic shoe box container, newspaper for substrate, water bowl, PRIVACY. Unfortunately, although they're beautiful animals, they just do not tend to be great display animals as juveniles. I'd start over small, get her feeding strong, and you'll find you move up in cage size rather quickly. Best of luck to you, and your new dumerils. Take care
Paul