Marc explained how this works to me. But the short answer is no, you won't have to have 2 data plans. What you do after adding the new line is ask them to do a device change. They'll move the Inspire from your new line to your current line. After they move the phone, you ask them to remove the data plan from the new line.
If they're well trained, they should already know the steps involved when you tell them that you're adding a new line to get the discount, but want the phone for yourself. But if they're not well trained, those are the steps involved. Although, if they seem completely clueless, you can have AT&T customer service take care of this for you.