1. Give each cast a name--such as the Beauty cast and the Beast cast, not cast #1 and #2 and label the rehearsals in that way as well.
2. If you have an odd number of performances, toss a coin for the extra show. Usually, I give the losing coin toss person all publicity engagements (TV. and radio interviews, extra public appearances, etc) This summer it turned out at the coin toss cast (for a Saturday matinee) was 50% of one cast and 50% of the other. The kids had never even rehearsed together, but it was wonderful and new interpretation and full of energy.
3. When rehearsing, take turns with blocking. So the Beauty cast goes first on Tuesday and the Beast cast goes first on Wednesday.
4. My kids enjoy double casting, because it eases the pressure of playing the lead.
5. I depend on double casting because I always have someone to rehearse with if the Beast cast member is absent and a stand-In in case of emergency. On several occasions, I've have the double cast member who wasn't slated to be on stage that evening portray the role because of a family emergency or illness.
6. Sometimes you may have to build two costumes for the part if the students aren't the same size, but we don't mind that either. We know that everything is for them and their opportunity to perform so that's something we accept right from the beginning. Usually, the kids are similar in size however.
There will always be some competition, but usually I split the strength between the two casts anyway and they set the benchmark for the others anyway. If you need more help, just contact me.