iMessage message routing
You are assuming Thing2's iPad and iPhone are both acting as 'Thing2'. That is only partially true. Assuming the iPad is WiFi capable only, it can only receive messages on its e-mail addresses (the address of the iTunes account or other e-mail addresses). The iPhone, however, can receive messages at its phone number or linked e-mail addresses.

Now when an iMessage is sent to a phone number, the message will only appear on the iPhone as the iPone is only capable of receiving those iMessages. The phone number is linked to the iPhone, not to the iTunes account.