Most reptiles are solitary and do not enjoy the presence of another in its home territory (exception of some species that are able to live in communities). So it doesn't matter if its alone or not.
For crested geckos you are able to house more than 1 esspecially in a 29g. Some tolerate another gecko, some don't. Depends on the gecko itself. If you have a female, and have no intentions of breeding its better to get another female. They can share the same enclosure. It's not that males can't be in the same enclosure as the female but of course... boy gecko + girl gecko = baby geckos :coverlaugh:. If the female is not receptive, the male will most likely stress her out. So in the end you'll need a separate enclosure anyway. There are some people who have females and males kept together throughout the year...I don't really have any experience in that so I'll leave others to comment on that.
There are possibilities that the female will beat up the new female or vice versa. But then there is also a possibility that they'll be best buds or just tolerate eachother.
If you intend to get another CG, please take precautions such as quarantine. And when you intend to introduce them after the quarantine period, be ready to intervene if things go bad. There was a thread about introducing two females together recently, should check that out for a lil extra info.
if my long reply confused you..sorry :blushing: