2 iphones both receive same messages and sometimes phone calls. What causes this?

If you are receiving imessages intended for other devices you can fix this by going to Settings>Messages>Send & Receive and do one of the following:

1. Uncheck the email address shown under "You can be reached by iMessage at" on both devices if they are iPhones; or
2. On one of the devices go to Settings>Messages>Send & Receive, tap the Apple ID, sign out, then sign in with a different Apple ID. Note: you can continue to share an Apple ID for the iTunes & App store for purchasing if you wish; it does not need to be the same as the ID you use for iMessage, FaceTime, iCloud and other services.

I am having the same problem. I think it is related to when we updated my wife's phone to the latest iOS (8.2). Our phones were working fine (i.e. separately) before. I don't have the problem with the messages. I have the issue where if someone rings my phone, both phones ring like they have been linked and vice versa. It was all fine before we updated her phone.

Phone calls are indeed handled by the carrier as Ocean20 suggests. But perhaps you're confusing a phone call with a FaceTime call, which would pick up your Apple ID.

Yes, you answered your own question. Both phones are registered under the same user. I recommend you each get and use your own user names.

As to message issue you need to have separate Apple IDs for iMessage. As to receiving the phone calls I would check that with your carrier