Hi there,
At this point, a vet visit is definately in order. Make sure you take him to a qualified reptile vet. You can find one by going here: http://www.arav.com/.
Have you seperated them? If you havent, please do so. A 20 gallon long will be fine for him until you can get him healthy again. The bigger one may be dominating the smaller one, to the point where he wont eat. Even though you have a nice sized enclosure for them both. Beardies are not very social creatures and prefer to be alone anyways, sometimes you cant get two dragons to live together and both be healthy. Do you know what genders they are?
Anyhoo, seperating them and getting the little one to the vet is what needs to be done.
Good luck with them! 
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~Melissa~
1.6.3 Leopard geckos
1.1 Bearded dragons
1.0 Hog Island Boa
0.1 Western hognose
0.1 Coastal carpet python
1.1 Crested Gecko
1.0 Savannah Monitor
0.0.1 Sulcata tortoise
1.0 Eastern Collared Lizard
1.1 Ferrets
0.1 Chinchilla
2.0 kitty cats
0.0.1 Firebelly newt that lives with a betta