Thanks to the fantastic Apple support - it was because there was confusion as to the apple id itself - one with just my "handle" the other with my full email address. It should be the same apple id for both devices. Once we fixed the original apple id which was confused with old itunes id -- problem solved and all devices are working perfectly!
Apple Support is amazing, patient and very knowledgeable! Woo Hoo!!
I have the same problem ie "How can I use iMessage on both my iPhone and iPad. iPad tells me "email already in use? " and i dont know how to fixed it.. Pls help!!
Don't tag your question/request onto a post that's about a year old. You usually won't get an answer. Start a New Discussion post with your own unique question.
Put a new email address - gmail? - on your iPad.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8