Definitely a chance to move up if your player is a stand out (but also depends on if there is room on the roster for the higher team, typically because someone moves down).
Double coaches usually has meant a head coach and assistant coach, both of whom work with both teams simultaneously. Frequently head will work with higher team, and assistant with lower team for certain drills, but the teams do spend a lot of time together with both coaches (or swapping coaches depending on drills). Also coaches back each other up, so if one is sick the other leads the practice, coaches the game, etc.