What kind of Iguana are they? A good resource for you is: http://www.iguanaden.com
You can tell the sex of an Iguana by their femoral pores on their thighs. The males Iguana has larger pores. However, I would suggest having a qualified vet determine the sex for you. I always thought that my Ig was a male, but later found out that it wasn't when I had her to the vet.
As long as they are not aggressive, it should be fine to house them together. Please also make sure that you are not feeding them commercial food. Though they will grow big quite fast, they will also die young from renal failure. They food that you buy from the pet stores was designed by Ig animal farms which sell the meat to 3rd world countries (they call it "bamboo chicken") so it is designed to grow the animal quickly, but they don't care about long term health.