My son is in 4-H, and it was not required that he have a pair of chickens, but he does enter both a male and a female. This way, both have the chance to be grand champion winners. He did enter some single chickens, as well.
If you have 2 hens of the same breed & variety, only 1 can win the big prize. If you get a pair, try to get 2 different breeds. They judge all the same breeds and varieties together.
In our fair, all chickens have their own cage, and all of my son's chickens were kept together.
This was our first fair and we both learned a lot. It is also a lot of work. His club had barn duty for 3 hours, 3 times during the week. They were also responsible for feeding the club's chickens 2 times/day. As well as, getting their chickens cleaned up for the breed show. They also had showmanship.
It is a good experience for the kids.
Hope this helps. Any more questions, let me know, and I'll try to answer them.