Q: How to cancel old IDs and create a new single one

I am 68 years old and I am using apple products since 1998. First I got my iPod, then my first iPhone, later another iPhone and two years ago an iPad. Somehow, I created two IDs. I have two Usernames (e-mail addresses) and use one single Password. I'd like to have one single ID and authorize my family-member to use the second iPhone. Until now, I always had to set a new Password whenever the other username was used. Now I want to by a new iPhone but under this circumstands I'm afraid to create even more Dilemma with ID's. Presently I am using my iPhone 5 (for my wife), my iPhone 5S  for myself and the iPad for all members in the family. The iPod is not in use anymore. Does anyone have an idea what I can do?