I guess it's more like when they merge, it depends on who gets absorbed. If Tae-oh gets absorbed into Tae-jun, then he'll be Tae-jun but with bits of Tae-oh. This means that whenever Dongjae is with Tae-jun, he'll be looking for traces of Tae-oh. And if Tae-jun gets absorbed into Tae-oh, he'll be Tae-oh with small pieces of Tae-jun. Dongjae will be looking for traces of Tae-jun. So Dongjae will always be reminded of the person that was absorbed, and the fact that the other "person" kind of no longer exists. That's probably what he felt.