Here is an article that deals directly with this question and can tell you how to link iMessage in both phones:
How can I use my iPhone telephone number with iMessage on my iPad, iPod touch, or Messages for Mac?
You can link your iPhone telephone number to your Apple ID for use on your iPad, iPod touch, or Messages for Mac by signing into iMessage with your Apple ID in Settings > Messages. Then sign in to the other device with the same Apple ID in these locations:
- iPad and iPod touch: sign in to iMessage in Settings > Messages.
- Messages for Mac: sign in to iMessage in Messages > Preferences > Accounts.
Linking your iPhone telephone number with iMessage is not available for the iPad (1st generation) and iPod touch (3rd generation).
Follow these steps for more information or if you are experiencing difficulties linking your iPhone telephone number to your Apple ID.
If an unknown device registers with your Apple ID and you did not initiate that registration, you can reset your Apple ID password to remove it. You will then need to enter this new password on all devices using iMessage or FaceTime.
Take care, and thanks for visiting the Apple Support Communities.