mixing species is very badly frawned apon with good reason. There are more cons than pros to adding more than one species in a set up.
Just a few...
*they may get stressed from having the other species so close by and not being able to escape that "territory" like they would in the wild.
*This is hard to explain, I hope I make it clear and if not maybe someone else can help out.... but some lizards carry different amounts of parasites or bacterias that do not affect them in any way but if a different species is introduced to that same enclosure these lizards may contract these parasites or bacterias and become deathly ill or worse. For example if you were to house a bearded dragon with a uromastyx. The uro may end up with a really bad case of coccidia as this is found in bearded dragons fecus and they do not have any side effect from it unless it is very high were as the uro would pick up this parasite and become really ill and need meds and quarenteen. Besides the fact that uros need a basking of about 120 and beardies about 105-110.
*As stated above, the two species may have similar but not exact housbandry set ups and this could cause problems.
*One species may be more dominant than the other eating all the food, taking all the good hiding and basking spots, and bullying the other species, possibly hurting or killing them.
Just to name a few. I'm sure others will add more. but I'm off to the gym now so have to flee.