Most likely they are being housed together, right? If you have them in the same enclosure, please separate them immediately! What always happens is one dragon thrives, while the other is stunted in growth, is lethargic, develops MBD, and is so stressed out because they are being dominated that they eventually die or they're killed by the other dragon. They are solitary animals in the wild, they only come together to mate, and this is why most babies in pet stores have toe and tail nips, and some are huge and lively and others are tiny, frail, and lethargic.
You need 2 of everything, enclosure,
UVB light and basking light, digital probe thermometer, decor, etc. And they need to either be stacked so they cannot see each other, or in separate rooms. Once you separate them the one that is not thriving will pretty much immediately start recovering, but as long as they're together you're going to have one sick, tiny dragon and one healthy, growing dragon. And when you move them, please move the healthy one, not the sick one. The sickly one is too stressed already and needs to stay in the home they know, while the other one will be fine when he's moved to his new home.
If you don't have the money for another enclosure and set up, you need to go buy a big, plastic tub, a clamp lamp, and a bright white basking bulb, and a basking platform for him. Set him up in the plastic tub, he can go without a UVB tube for a week or so. The sickly one cannot and needs the UVB tube.