You can keep several animals together in one terrarium. However, this depends on a number of things.
The first factor and also one of the most important are the animals themselves. The animals you put together may simply not get along.
Something that we see in humans and animals and that you will have to accept in the unfortunate event that it happens. The animals must be separated in different terrariums and cannot be together.
In many cases, Crested Geckos will be reasonably tolerant towards each other, however it is not smart to keep several males in one terrarium (this often causes problems during the mating season) several females is often not a problem.
In addition, it is important that the terrarium is large enough, the animals all need their own hiding place to rest during the day or to withdraw when they want to.