Depends on what you mean by giant gecko. If you are talking about R. leachianus than definately not. Leachie's eat crested geckos in the wild so I'd strongly advise against it. If you mean a giant day gecko than that would probably be fine. As with any animal, more species/individuals can mean more problems but a giant day gecko and a crested gecko could live together as long as there was enough space and cover for both animals.