IPhone – My iPhone 4 won’t send text messages!

To address this issue I'd recommend checking the iMessage settings on both phones. If they're both using the same Apple ID for iMessage, they may have trouble sending to each other. Changing the iMessage Apple ID will not mess up anything else (like the Store, iCloud, etc).

To check it, go to Settings > Messages > Send & Receive

A button at the top of that screen will show you which Apple ID is being used for iMessage. If they're the same on both phones, you'll need to sign out on the one you wish to change. To do this, tap the "Apple ID: " button and then choose Sign Out.

Then you'll be able to sign in to iMessage with a different Apple ID, existing or new. I hope this helps!

iMessage is actually an instant messaging service, like Google Talk or AOL Instant Messenger, except iMessage is made by Apple and is only for Apple products. Only an iPhone with cellular can send SMS messages as well, and can automatically associate phone numbers of registered iMessage users with iMessage accounts, so if you try to use the app to send a text to a phone number registered to an iPhone, it will send an iMessage instead. iPads cannot send SMS messages to non-Apple devices nor an iMessage to a phone number not registered with iMessage from a Wi-Fi-only device.

http://support.apple.com/kb/HT3529