Are they females? If so, then yes, you can house them together. However, you have to quarantine them, watch for bullying, and make sure they are not fighting over food. They also have to be similar in size.
Exo terra terrariums tend to be bigger in height than in wide. The floor space is really minimal and seems to be more suitable for arboreal animals than terrestrial, dependent on the size of the exo terra tank. A 18x18x18 seems better suited for one gecko to me. I use to keep one of my geckos in a 18x18x18 exo terra a couple years ago and found it to be minimal space for one gecko and not enough for multiple geckos.
Yes, if both females are about the exact same size, both have been quarentined for a minimum of 90 days, both have had proper fecal exams, and as long as they in no way bully each other. They live best on their own. They do not want nor need a friend
I agree. I have an 18" x 18" Exo Terra tank, too. It's too small for more than one leopard gecko because there isn't enough room to put in the required additional hides. I house multiple leopard geckos together. They rarely share the same hide and prefer to be in different hides.
Chakra had a similar tank with similar floorspace before I set up my homemade rack system. Long story short I find it cramped for even just one adult.
I also, although its technically possible with females have seen too much go wrong when housing leos together---anything from cannabalism, bullying, illness, injuries and almost death. Thus, house them alone. They will be happier that way.