introducing leopard geckos

Question
I have two leopard geckos that I want to introduce to each other. They have been living separately, and one is very active, and the other is much more passive.  What is the best way to introduce them to each other?  How can I read their body language to tell if they are getting along, what are signs of domination or stress, and what should I do if that happens?

Answer
Hi Julia,
First important thing is...what is their sex?  If they are both males, they cannot ever be introduced or live together as they will fight til death.  Male and female generally can live together, but then you have breeding issues which can be very stressful on both sexes.  Two females..may be able to live together provided the area they are in is large enough for two of them...nothing smaller than a 20 gallon long tank(I prefer larger for two) needs to be used.
IF you have two females, you can start by putting them where they can see each other while in their tanks.  Then, when you see things are OK, you can take them to a neutral territory...place the passive one there first, then introduce the other one to that area.  Watch for charging, hissing, standing up high on all four feet...The passive one may submit, laying flat, turning her head away, etc.. If this happens be ready to intervene and chances are even if they are females, with their difference personalities they will not get along.  Your passive one will be severely stressed which can lead to illness.
If the meeting goes well, I would use items from the passive leos tank, and completely wash anything you use from the aggressive leos tank before placing it in the "neutral" tank so that the aggressive one thinks that this isn't her territory any longer.  Do be careful as you may find that the more aggressive one wants to fight to regain territory.  If that's the case, or if the first encounter doesn't go well, then chances are, they cannot nor should not be housed together.  
With the neutral territory meetings, you can do those over time to see if anything changes as to them getting along...in all honesty, leos really do prefer to be alone.
More signs of stress would be not eating,not hunting, not wanting to come out of her hide at all.
If you find that they can be housed in the same tank, be sure to provide hides, humid hides, etc for each of them.