This is an old post so I'm sure that you've gotten everything figured out by now, but incase you havent:
First just let me say that cresteds are awesome! Mine is sitting on top of my computer while im typing this.
Anyways, 50 gallons is fine for two of them however with any animal, you'll want to make sure they are not both males. When determining size with a gecko such as a crested you don't necessarily want space width wise, you want space in height. Crested geckos like going to the highest point of wherever they're at. Its where they feel safest. Plants are good for cresteds for both cover/hiding and climbing, the best kind of decor is of bamboo. Cresteds do not require any extra heat but I find that a small heating pad helps, sometimes you get cold, so do they. When buying anything from a big chain store, you're correct, quality does tend to suffer. However geckos and reptiles in general are pretty hardy so its not as much of a concern, for the best results I recommend going to either a breeder or a reptile convention. Sexing them is fairly easy, males have bulges under the beginning of their tails ( testicles ) and females obviously do not have these. Since you will be housing multiple geckos in the same enclosure, make sure you're supplying enough food. As for misting, I usually mist mine 2-3 times a day, I've noticed that crested have an issue with shedding on their feet, and higher humidity can sometimes help with this.
Also just in case you didn't now, unlike other common geckos such as leopards, the crested geckos tail will NOT grow back if its dropped, A.K.A frog-butt. When handling them be very cautious because they like to jump. Always handle them close to the floor in-case they over jump onto your hand or arm, etc.
Hopefully I've answered some questions!