I bought two iPhone 5s, one for my wife and another for me, and put them both under the same Apple ID. This resulted in a lot of trouble as follows: when I signed-in at iCloud, and then used FaceBook, and Messages, my Apple ID was showing on both iPhones. If my wife would turn her iPhone off, or didn't have cell service at some remote location and I would text her a message, this message would come back to my iPhone, my laptop, iPad, and my hotmail address. Not only that, but all of my contacts would show on her iPhone along her contacts, and so the contacts on my iPhone. Any changes she would make to her contacts would affect my contacts, and vice versa
Talking about a big mess!
Anyway, I had a chat with an Apple technician and was advised to logoff iCloud on both phones, to remove the checkmark on both my phone number and email address in her FaceTime settings, and also to change the settings on iMessage. The next step was to create an Apple ID for her. Now we just share apps and the rest under the "Family Sharing" option.
Anyway, I decided to stay off iCloud all together, and back-up the iPhones manually, mine on my laptop, and hers on her desktop. I back them, and if I need to sync contacts and the rest, I do that manually.