All gecko species are very territorial. Males won't tolerate each other. There are a few species which are exceptions, such as tiny geckos of the genus
Tropiocolotes with which it's possible to house several males together, otherwise for any other gecko species it means one of the males will be the dominant one and will sooner or later kill the other males housed with him.
Besides, both cresteds and gargoyles prey in the wild on smaller lizards, including individuals of their own species! Cannibalism is a common thing among geckos when there is a size difference between individuals which makes it possible. A baby crested is likely to be eaten by one of its parents in no time.
One of the basic rules is one species= one enclosure. Don't mix species. Species from different parts of the world may contaminate other cagemates with diseases they carry on them. They are "healthy carriers" of diseases which can be fatal to geckos of different countries or continents.
Cresteds and gargoyles are also closely related, that means attempts to mate with each other may occur with a group of adults. As females of one species will not breed with a male of another species, it may end up in a fight, with dropped tails (cresteds never regrow tails once they're dropped) plus more or less serious wounds.
If you don't have enough room/money for 2 complete setups, 2 terrariums, and both species, then you have to choose the one you prefer. Otherwise, each species will be fine in separate enclosures; keep in mind they don't live in the same type of environment. To make things simple, gargoyles need a drier environment (yet not a desert-like one!) than cresteds and also prefer bushes with small twigs whereas cresteds will rather rest on stouter branches or on cork bark. As a consequence, their setups should be arranged a bit differently.
