When I had my Iphone 4 whenever I texted via Imessage the text thread would also show up on my Ipad with the same account. Now that I upgreaded to Iphone 5 this no longer works. Help!!!
To help with your problem, I'd recommend you make sure that both your devices have your iCloud (or registered email) and phone number active to receive/send messages. What I mean is this:
Go to settings --> messages --> iMessage then in the "send & receive" make sure the apple ID is also enabled (if you wish to use it to send/receive messages that is). Do the same procedure on both devices and if at first that doesn't work, log out of everything (iMessage, facetime, iCloud) turn the devices off and back on and then reactivate everything again.