It's likely a combination of issues including lighting and them competing for resources. I think Petco is still having their $1/gal sale so it's a good time to pick up a new tank. You may want to consider picking up two 40g breeder tanks ($40 each instead of $120 each) one for each of them. This will be big enough to last into adulthood. I'd pick up two Zilla Slimline fixtures (T8, 18'') and two ReptiSun 10.0 UVB tubes (T8, 18'') and some small command hooks (1lb rated). You can mount the fixtures by putting the command hooks through the holes in the back of the fixture to mount it just under the top of the tank. I'd also pick up another basking light to use in the second fixture (you won't need the coil
UVB light anymore). If you don't have a digital thermometer I'd pick one up. An infrared temperature thermometer is probably a good bet because you can share it between two enclosures. Petco may have ZooMed ReptiTemp available which will do. An adjustable lamp stand to hang the fixtures with basking lights may be helpful for getting the right temperatures in the enclosures. Having their own space and proper lighting/heat should do wonders for their overall health. Even if you don't go for the two 40g breeder tanks now, I'd definitely get a second 20g tank and second setup along with the two UVB tubes for now.