IPhone :: Combine IMessage Sent To Phone Number And Email?

I talk to someone who sends me imessages to my phone number from their phone, and also from their ipad to my email. Both phone number and email are under the same contact. But the mesages go into 2 different threads. Id like to combine them since being under the same contact they show up exactly the same in the messages list.. so i cant tell if im messaging the ipad or the phone..

ideally id like to write a message to that contact and it would go to both their ipad and iphone.