I would recommend to wait until they are both full grown before housing them together if it is not the case already, to prevent bullying which would lead to one animal not eating and not growing.
Once adult I will also recommend to separate them for a few months each year to give the female a break.
I wouldn't keep 2 adults in that tank. 12x12x18 = 1.5 cubic feet = 11.2 gallons, and IIRC it's recommended to have at least 10 gallons per gecko. An 18x18x18 (25-ish gallons) would be a more reasonable plan for a pair.
Mountain bikes are for slow people, and reptiles are far better pets than cats & dogs!
I give my cresties at least twice as much room as "recommended" on the 'nets, and they seem to do better for it. Frankly I consider a 20G high turned on end to be a standard volume to house a single gecko.
I used to house two geckos in a 2'x18''x2' and that was a great size for a pair.
However I have since moved all my crested geckos into single-living, only pairing up temporarily for a few days at a time for breeding purposes. This helps me monitor food intake/output much better.
Yeah, I feel about the same way you do, mainbutter - my lone female crestie is in an 18x18x24 by herself. I know she doesn't *need* that much space, but she sure uses it - she's all over that cage.
Mountain bikes are for slow people, and reptiles are far better pets than cats & dogs!