Best way to do this... I'm kinda like onto stuff like this
If you 2 are on the same account.(which if you are married I assume you are) or if not I'm sure you know her last 4 SS#. Get a used Sprint cell phone, call sprint have sprint swap that phone for your wifes. Sprint will have you enter all the info (MSL code, etc.)
A few minutes later call back and put your wifes phone back on her account. Even though you cant call or respond to any text messages, Sprint will continue to send incoming text messages to BOTH phones!!!! I know this becasue I have had many cell phones and everytime I change to a new one, I continue to receive text messages on both phones for a few days. Good Luck!! email me with any questions.