I don't really encourage the idea of splitting the band into training band and performing band.
By doing so, the training band members will feel neglected and will not have a sense of belonging. As a result, the band's morale will suffer greatly.
After splitting, the performing band will improve faster and the training band will not improve as fast as the performing band. This creates a big gap between the performing band members and the training band members. After the performing band members graduate, who will take their place? The present training band members? Will they be up to it if they are neglected in the training band? Therefore splitting the band is not a good idea.
Also, after splitting, the members of the different bands will spend less time together and feel less united as a band. As mentioned earlier, the training band members will feel neglected and will not have a sense of belonging.
Although splitting the band means that the performing band can play more pieces and provide a more interesting repertoire for concerts, it is not worth sacrificing the future of the band for it.