ok..well to start, dont house them together, im sure you knew that tho.. i never had a problem fatteninng spike up...when he was young i would feed him all the crix he wanted 2 times a day and boy did he eat a lot..and he would eat a few bites of greens, but not too much..then once he hit 15" i started with superworms and have been feeding those as a staple ever since. he is now 1 1/2 yrs old, 20" and a fat little thing

also try silkworms and hornworms, stay away from all worms except these 3(never take horn(tomoato) worms out of ur yard) as for cage, i built spike a 4x2x2 cage out of melamine, with slate for a substrate. you dont need to use mealamine, but i liked the white since it helped brighten up the cage. beardies thrive on a bright environment.

and a recent pic of his fattiness
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1.0.0 Bearded Dragon, Spike
1.0.0 Cuban Rock Iguana, Buddy