Generally no. It is possible but it is extremely easy to kill one or both of the animals. Even very experienced keepers can miss the subtle signs of stress before it's too late. Do a search on the site for more info.
I believe, if you have to ask this question, you dont know enough/have enough experience about them to do it.
No, they should definitely not be kept together. I'm not trying to be harsh but I am going to tell you all the reasons you will not want to keep them together so you know the reasons behind it.
Keeping two chameleons of different ages is a huge risk to the younger one because the adult will be faster to get food, much more intimidating to the younger one and could possibly be harmed. They are not social creatures and don't like others in their space. Even if they are not actively fighting they can intimidate each other very easily. The little female may be under constant stress and not get enough food or light if she is too scared, or scared away from the basking area. Not only would it be very stressful for the younger one to get food and not feel intimidated by the adult, which as a male will probably be very territorial, but you would also most certainly end up with an immature animal forced to mate too young and try to grow fertile eggs at the expense of her own health.
It might be okay to maybe attempt cohabitation if the female were the mature one, but as a juvenile you are asking for disaster. Producing fertile eggs is a huge toll on the body of a female metabolically and nutritionally, and right now she has higher needs than an adult would already because she is still growing herself. Eggs always come first when it comes to using nutrition resources so her own growth would be jeopardized if she was mated too young. She wouldn't just end up smaller, her bones may not properly form, which will cause many problems later in life.
In this case I would say it is absolutely inappropriate to house them together. It will probably not even be appropriate to house them together when they are both adults because veileds tend to be pretty territorial and the stress of constantly "battling" each other through intimidation if nothing else can be very taxing on their health. And the male would probably constantly be trying to mate with her. When they are gravid they do not like to have a male around and will attack them. So even if they were peaceful normally you would need to separate them when she was gravid - which you wouldn't be able to stop if they lived together.
Two cages, two complete setups if you have two chameleons. They will be much happier and healthier.