The difference between treating a patient in one phase versus two phases is the increase in cost and additional appointment visits required. University research and collaboration with orthodontic study clubs has verified this. In the 1980's, 40% of the orthodontic patients were treated with two phases, but now it is only 10%. Thus, if the orthodontist can give the patients the same excellent results incurring less expense and requiring fewer appointments, then two-phase treatment is not for your child.
During the beginning years of my career, two-phase treatment was utilized until the research, my clinical observations and orthodontic technological advances allowed me to treat the same patients with one phase. If your orthodontist recommends two phases, it should be for very severe cases and crossbite correction. Your having knowledge regarding the use of a particular treatment will inspire confidence in the course of treatment selected.