If you have a computer with ant+ sensors, then just use your wife's phone.
There is a free IPBike app on the google play store that will do what you need. It will also display a map for navigation.

I have a Garmin 1000 with its ant+ speed and cadence sensors, but any ant+ system will work. Both my computer and my wife's phone can pick up the sensors simultaneously. I can also see my heart rate on the garmin and she can see her own on the phone. She normally carry her phone anyway so I just add a little mount for it on her handle bar.

CJ