I'd make very sure the new little guy is acclimated (oh yeah, congrats!). Then introduce them after that. I included some notes I put together some time ago about acclimation and introducing. These are just my thoughts, experiences, and beliefs...not rules or anything like that. Just hope it's helps.
Acclimation (I know, you've done this before):
Acclimation is very important to the success of your uro becoming and remaining healthy. As these reptiles are most likely wild caught (which species will have a lot to do with that) they are going to be very stressed once they get into your care. They may not eat for some time, maybe up to 3-4 weeks. They can be very skittish as they are already not "sociable" animals. This is why it's very important to have the habitat already set up. Some things to help with the acclimation period, would be to set up the tank in a low traffic area of the house. Covering the sides so it cannot see out has been proven effective. If the enclosure is glass, a decorative covering (much like those used on the outside of fish aquariums, but with a desert layout) on 1-3 sides can help it to distinguish where it can't go. Changing the food out while it's sleeping also helps. There's many things that can be done and everyone finds things that work well for them.Housing-Uromastyx are generally non-social.
Two mature males may not be kept together! Sooner or later one will attack the other, possibly causing serious injury. Females vary greatly in temperament. Most get along fine with a male, but I have heard of exceptions. Uros are by nature territorial, and even calm animals tend to attack new individuals place in their cage.
Speaking of cage size, the larger the better. At the very least, the cage should be 200% their length deep, and the length should be if possible, 300% the length of the Uro. It's been said that uros are not climbers, but spend any time watching mine, and you'll change that opinion. Again, these sizes are just suggested minimums, and are not intended to be any more than guidelines. My terrarium is 48x18x18 and provides enough room to allow him to scoot around and climb. Remember, the more room they have, the better. Uros are desert lizards and are burrowers by nature. Lots of rocks and a suitable substrate is important. Rocks will help keep their nails trimmed as well as giving them a comfortable place to bask for heat. It is also important that the Uro has a place to sleep and provide a stress free atmosphere.
This can be provided with some kind of hiding place, such as a cave. Such things can be purchased or made by appropriately stacking rocks. If this route is taken, it's very important they're anchored or glued, as to not collapse (using a non-toxic adhesive) on the Uro causing injury.
Introducing Uros
1) Get a fully set up and ready tank for the new arrival. Get all the temps and such right.
2) Get the new arrival acclimated to where eating is regular and gaining weight as needed.
3) After that, maybe move the tanks into the same proximity so they can see each other, and monitor them for any eating/appetite changes. After a week or two, if all looks good, try and introduce them.
4) If your going to use one of the existing tanks as their new home, you’ll need to make it a “new” home.
5) I’d remove the uro from the tank, and then take the opportunity to clean it out, including changing the substrate. A spray and wipe down of a mild bleach and water solution wouldn’t hurt.
6) Rearrange the “furniture” so it really looks like a new environment. This will help eliminate any territorial issues from the one that was there originally.
7) Make sure there is at least one hideout for each uro, though three would be better (more if space allows).
8) Provide at least one food dish for each one as well as a separate basking spot for each.
9) Finally, if all goes well initially, I’d definitely watch and monitor them closely. Again, feeding and weight gain/loss. Sometimes, aggression is very passive and notice so noticeable. Watch for sores or thickening of skin in centralized places which can indicate one uro continually biting the other.
Hope any or all of these help.
I'll be waiting for pictures.

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2.1 - Rocky, Runako, and RoxyIII (my care sheets)
