Hey breee25!
It sounds like you two may be using the same Apple ID for your messaging, which would lead to this issue. I have some information for you regarding this issue:
Additional Information
- You can change the address that new iMessage conversations are started with in Settings > Messages > Send & Receive > Start new conversations from:. Note that this setting applies only to new conversations. If you would like to change the address from which messages are sent, first change this setting, and then delete the existing conversation and start a new one.
- iMessage responses will be sent from the address the recipient most recently messaged. For example, a friend sends you a message to your Apple ID. Responses in this conversation will be sent from your Apple ID, even if you've selected new conversations to start from your phone number.
iOS: Using Messages
http://support.apple.com/kb/HT3529
Use the information in that article to view the "Send & Receive" settings in your Messages app on both phones and adjust according to your desires. Thanks for being a part of the Apple Support Communities!
Regards,
Braden