Your dragon is very stunted (at 4 months old he should be much longer and larger) due to a lot of problems here, the biggest one is that you cannot house dragons together, you just can't. This is ALWAYS what happens, one dragon thrives while the other becomes stunted, stops eating, is lethargic, usually develops a calcium deficiency and MBD, and eventually dies. It's due to the other dragon totally and completely dominating him, taking all the best
UVB light and basking spots, all the good food, and constantly exerting a continuous stress over the submissive dragon.
If you're going to breed dragons then you need to have multiple set-ups for the ones you're going to keep ready to go, they need to be separated at around a month old and in their own, individual enclosures/tanks at that age at the oldest. Another issue is that a 20 gallon long tank is no where near large enough for a Bearded Dragon, and this is not helping the issue for either dragon; keeping 2 4 month old dragons together in such a small tank is magnifying the problem significantly. At 6 months old a minimum tank size is a 40-gallon breeder tank (longer, lower, and wider) for EACH dragon. And at a year old and older they EACH need a 4'x2'x2' enclosure, again you can get away with a 40 gallon Breeder tank for each one for a time, especially the stunted ones, but this is not a good situation at all.
Also, I didn't catch what
UVb light you're using, how far away it is from their basking spot/platform (this is a very crucial problem to keeping 2 together, they can't both bask under both lights at the same time after eating), whether or not your
UVb light is obstructed by the mesh lid to the tank, and how old it is, but you need to be using ONLY long, flurescent UVB tubes in long tube fixtures that have a metal reflector inside them, and they must be at least 10% UVB tubes (or 10.0). You cannot use a compact or coil
UVB light for Desert Reptiles, and if you are then they aren't getting any adequate
UVB light, and if you are using either a compact or coil
UVB bulb or a long UVB tube that is the weaker T8 strength and not the more appropriate T5 strength, you must poke holes in the mesh lid and strap the entire tube fixture to the underside of the mesh lid, over the Hot Side of the tank. T8 UVB tubes must be within at least 6" of the basking spot, unobstructed by anything, and replaced once every 6 months. Again, if you're using any type of compact or coil
UVb bulb, even a Reptisun 10.0 UVB compact or coil
UVb bulb, this is another reason for the stunting, the lethargy, the lack of appetite, etc.
Please get them separated ASAP, get them both adequate UVB tubes, and get them both at least 40 Gallon Breeder tanks, or even using a large, plastic tube that is at least 50-60 liters, like the kind you can buy at walmart, then putting the tube fixture across the tube and putting their bright-white basking bulbs in clamp lamps that are clamped to the top edge of the plastic tubs, with basking platforms that are high enough to get them within the correct distant of the UVb tubes and get their basking spots between 105-110 degrees (surface temperature, must be measure with a $10 Digital Probe Thermometer, not a stick-on).