Hello and thanks for researching this very important question!
Reptiles, by nature, are solitary creatures that only get together to reproduce.
Even males and females living in communities for security reasons do not spend a lot of time in each other's territories.
New individuals arriving in the community are a source of stress and competition in the wild...this is the same in captivity.
Additionally, the Uromastyx family of lizards contains several sub-species that would not recognize each other in a social setting.
http://www.kingsnake.com/uromastyx/
Competition can be severe enough to cause physical injuries from fighting, or starvation from food-guarding in smaller animals.
I do recommend that you get a separate enclosure for the new lizard to prevent one of these tragic occurrences.
If you need additional support at this time, please click "Reply".
Retired Veterinary Technician
Lhasa,Shih Tzu Breeder/ B.A.Neurophysiology & Animal Behavior/I use plain English!