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The easiest way to use 2 devices under 1 appleID is to setup a second email address. I had to add the email first on apples website but once it's confirmed you can then use FaceTime/iMessage without problems.

I have my own domain so use iphone@mydomain for the phone and iPad@mydomain for the other and obviously you'd send a message to ipad@mydomain from the iPhone.

This also works with FaceTime as I use for a remote camera :)

To add emails, go to http://appleID.apple.com and there is a section for alternate email addresses.
 
Working fine across my wife's iPhone/iPad and my iPhone/iPad. The key is definitely use email addresses. My phones kept trying to send on behalf of their phone number and so then the iPads wouldn't get the messages. Finally I just deleted all conversations and started them via the iPads which created message threads on all 4 devices. As long as we keep using those conversations we are set. Really awesome!
 
Working fine across my wife's iPhone/iPad and my iPhone/iPad. The key is definitely use email addresses. My phones kept trying to send on behalf of their phone number and so then the iPads wouldn't get the messages. Finally I just deleted all conversations and started them via the iPads which created message threads on all 4 devices. As long as we keep using those conversations we are set. Really awesome!

That's what I'm hung up on. I can message my phone from the household iPad just fine, and once the message has been received, I can continue a conversation between the two devices.

What I can't do is message the iPad from my phone, because the iMessage address only allows names or numbers. There's no way to give it the apple ID of the iPad as it wont accept email addresses as a contact name.

I was hoping this would be more of a hybrid between IM and SMS, but clearly none phone iOS devices are intended to be second class consumers only.

That's a shame.
 
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