Manage what you share to the cloud

You cannot each selective choose what to share. You can simply disable some things, like contacts, notes, email and so forth with that iCloud account. But either of you can also re-enable those whenever you wish.

AppleIDs are not meant to be shared. When sharing an AppleID you share a single account for every Apple Service used with that AppleID. There is no way to share an AppleID and have privacy of anything in the accounts linked to that AppleID (that goes for iCloud as well as FaceTime, iMessage, purchases, these forums, etc). Everybody with the AppleID password has full access to and control of each account using the AppleID.

You can turn off some items on a device via Settings > [username] > iCloud to prevent them from syncing to iCloud -- but having said that, sharing an Apple ID is not recommended. Normally a better approach is for each person to have their own Apple ID and then share selected things via iCloud - Family Sharing - Apple