My son is 5 and just started his piano lessons. He usually practices both hand together when he got a new piece at home by himself (I didn't tell him what to do and that is his own choice). But his sight reading is not that good yet, so during the class, teacher tried to practice with both hand with her help for telling him which not3es he needs to play. Sometimes, she will tell me to let my son practice separately first at home. I think it is just let him to familiar with the notes. It depends on how old is your kid and how good his sight reading is. After training, they should be able to practice both hand together for new piece. Maybe your kid's teacher just think your kid is not ready yet.