You need to use separate iCloud accounts. This will mean that you can both have your own separate contracts and calendars and put an end to them being deleted by each other.
Whether or not you have a separate iTunes account, is up to you. If you have a separate iTunes accounts you will not be able to access the music, books, apps that have been purchased by the other. This may be exactly what you want, but it also means that you both may end up purchasing the same item.