IPhone – Sending/Receiving SMS/MMS on iPhone 4s

As you can see in the first screenshot, you can set a number of email addresses by which you can receive messages at on your phone. However, as the second screenshot shows, you can only have one Apple ID connected to a device at a time.

You might be thinking, "Ok, can't I just add her Apple ID email address to my phone?"

The answer is no. Any time you add an email address to your iMessage account, the email is verified with Apple. If you attempt to add an email address associated with another Apple ID, Apple will decline it and neither iPhone nor OS X will not allow you to add the email address.

Conclusion: Unfortunately, there is no way to do this with iPhone or OS X. You could do what was previously suggested and set up a secondary account on your Mac (if you have one) and link that account to her Apple ID and use Message there to view the exchanges, but to specifically answer your question, OS X does not accept more than one Apple ID for iMessage per user and iPhone does not accept more than one Apple ID for iMessage either.

I'd try to Reset Network Settings, as detailed here: Not receiving Text Messages after upgrade to iOS 7 -- this seems to cure a wide range of ills for Messages.