You don't have to buy 2 packages to cover the entire family unless there are adults in the other stateroom. Even then, just make sure someone in the purchasing stateroom is in the photos being taken so they can be tagged. The package includes all of your children under the age of 21 regardless of what stateroom they are in. When you get on board, go to the photo people and have them link the staterooms.
They will sometimes offer a discount on-board but from what I have seen, it's still been cheaper through the planner.
For photos it has to be done via the photo people on-board