Ouch!!... that's a very good question.... Where are our Reptile "consultants" ~T~??... havent seen him on lately... anyone else able to help Marine123 here??
There is no way a crested gecko and a chameleon can live together, they have extremely different requirements meaning neither can live in each others habitats.
Crested geckos need a very damp but cool environment, no hezting needed but they do need a low percentage UV buld as well. Very easy to keep if you have the time... They need spraying every day to keep damp,sometimes twice a day in the summer and they need to eat a mix of live crickets, locusts and a premade mix food for cresties (bit like babyfood) that includes their vitamins, nutrients and fruit.
By Chameleons, i assume you mean the Yemens (or veiled) Chameleon as they are the most common and are the cheapest (at £60 for a baby) but they grow WAY too big for your vivarium and are much harder to care for. The chameleons need a hot heat buld to bask under and a high percentage UV bulb, this means to keep them damp you either need an expensive automatic misting system or someone around to spray them 3-4 times a day. This makes them high mainenance! They also need live crickets and locusts every day.
As a beginned, which i am just assuming now based on the very basic questions, i would go for a Crested Gecko after some more work, Chameleons are not a first reptile by a long way.