Cage Capacity and Colony Mix

A ten gallon aquarium can house one to two geckos (optimally one), while a twenty gallon aquarium (choose long aquariums over tall ones, even if they have the same capacity) could house anywhere from two to four (optimally 2) leopard geckos, all the way from hatching to their adult size. The exact number will depend on the temperament and territoriality of the individual lizards in your cage.
Don't make the mistake of assuming bigger is better and get one that's too big, however. The size of the tank could make it hard for the geckos to find their basking spots and hide boxes. Larger cages are still generally better than smaller ones, and will provide a better temperature gradient. Just don't overdo it.
Leopard geckos can be kept alone or together, and will not experience distress if kept by themselves. Groups can be a problem in the wrong combinations, however. Males often fight when housed together, and females sometimes do.
Sometimes, in a group cage, one gecko will grow larger and more quickly than the others, out competing them for food. If this is the case, the smaller cage mates will be undernourished. To prevent this, separate the largest animal and see if the situation improves.
Some leopard geckos are aggressive enough to need to live alone for the entirety of their lives. As these are not naturally social animals, the gecko will not be harmed by this kind of living situation.
Groups of females usually do well. They do sometimes fight one another, but this is much less common than it is with sexually mature males. Be ready with an alternate housing solution if any of your lizards become aggressive, however. Some leopard geckos just aren't made for a communal living situation.


FROM WALTER K.--
Adding a female to an enclosure that already has a female is a bit risky but one can minimize the risk by taking these simple steps-

1. Move the exiting female out of the enclosure for at least a half day (you can keep her in a ventilated plastic container). This will get her out of her 'comfort' zone
2. Move the decorations around. Again, this will get the existing female out of the "my house!" mode
3. Introduce the new female into the enclosure and let her get comfortable (about a half hour should do it)
4. Introduce the existing female into the enclosure and at the same time, add food (crickets or mealworms). This will direct both their attentions to eating and not as much to each other

Take these steps and you should guarantee a successful introduction.
Source(s):
http://www.supremegecko.com