Bam is emmacipated, that is to say very skinny. You can easily see his hip bones in the second picture, his arms are like stinks and his head fat storage areas are nonexistent.
Personally I would house Bam alone completely till he is fully recovered from being half starved to death by the pet shop. Having another dragon in the same cage could put more stress on him, even if that dragon was a cage mate before. Also, it is much easier to monitor the new comer's health, how much he eats etc, without another dragon in the same cage. Also, it will remove any risks to Gizmo, who though only a few weeks away from the petstore himself, could become infected with parasites, should your new dragon have them. Bad husbandry conditions and stress can cause severe flareups of internal parasites in a dragon.
Feed Bam small crickets, waxworms, phoenix worms (these are great little fly larvae newly available on the market, they are small soft bodied, high in calcium, and very lively), small silkworms etc to get his weight back. Make sure he is well hydrated as well, give soaks in luke warm water (about 75F), use a needleless syringe to offer water mixed with either store bought electrolites, or diluted gaterade to help stimulate your dragon to drink plenty of fluids. A form of pro-biotics could also be offered as well, or if you can get it, parazap may help too. A fresh fecal should be given to your reptile vet so he can test for internal parasites as well.
In the separate cage, get a fresh UVB bulb and heat lamp with proper temps etc. Pretty much leave him alone except to feed, water and soak. I suggest offering soft bodied insect foods (silkworms, waxworms, pheonix worms etc) in small amounts several times a day for the first week, then increase the amount offered. Once he is eating regularly and all you offer, you can move him back onto crickets, silkworms and pheonix worms with some greens as well. Wax worms should not be given as more than a treat normally but to help your new dragon put on some weight, giving them now would be beneficial but cut back after a week or two after he starts to eat regularly and starts to fill out more.
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PHLdyPayne