The bearded dragon could stress the other reptile out, injure the other animal, or vice versa. Bearded dragons will fight ruthlessly for their territory and would probably kill its housemate. Reptiles DO NOT enjoy the company of other reptiles. The two animals could also spread pathogens or parasites to each other. It's just not a good idea. There's no such thing as a 'community tank' for reptiles like there is for fish.
However, you could place a divider in the middle of the tank and put an animal on each side of the divider.