Its almost certain that they would mate as long as they are old enough. They will mate every time that the female is receptive after that too.
If you put two younger ones together they will likely mate as soon as they are sexually mature...which I don't think is good for the female. You are making her put nutrients into the eggs that she likely needs for her own body.
Also, keeping two together past the age of about 3 months will put a lot of stress on them. In the wild, if one chameleon is in close proximity to another either can move away if its bothered by it. In captivity there is not usually enough room for them to get away from each other. The signs of stress are often subtle...but sooner or later is almost certain that one or both of them will start to have health issues.
Once the female becomes gravid, she will repel the male. If he is there constantly, she constantly has the stress of repelling him to cope with.