That's because he has multiple emails/phone number linked to his iMessage account. When you sent to his iPhone you may have used a different contact email or phone number than the one he initiated from his iPad.
Thread 1 is linked to his iPhone number probably.
Thread 2 is linked to his email address, which is enabled on both his iPad and iPhone.
Which just goes to show you, it's not a seamless integration of the messaging system. I'd expect that if I send an iMessage to a ID phone number, it goes to all of his accounts. There should be a way to link an iPhone phone number to an iPad account.
Also, the same guy tells me that when he sends himself an iMessage from his iPhone to his own iPhone number he never gets it. I on the other hand, I always get it when I send myself one from my iPhone to my iPhone number. There is definitely something odd about the implementation of iMessaging, since the behaviour isn't the same for both of us.
I will sit this one out, until all the kinks are worked out. SMS is fine for me at the moment. It Just Works™.
Why would you expect him to get the same iMessage on two different iDevices?
Because that's how it's supposed to work. If you register multiple items to one address, ALL of them get the same messages. Or at least they're supposed to, but judging by reports, it doesn't seem to happen consistently. The example I gave above about people not receiving their own iMessages to themselves illustrates the potential issues.
Another guy on another forum is saying that when he sends himself an iMessage from his own iPhone to his iDevices, his iPad gets it but that very same iPhone never does despite having both devices registered with the same email address. That's that doesn't induce confidence.