Is it even possible to seal people into other people? I know the tailed beasts can be sealed but I thought that was just because they were essentially manifestations of chakra. I don't think you can seal actual people (only their chakra) into others, right? So for example, when Sasuke went to see Kurama he was still present in the outside world, only used his Sharingan/chakra to see what's inside. I mean, Danzo tried merging with cells of the first Hokage, but that was not what is meant here.
But if it were possible to seal people, I think there could be some crazy stuff happening. Technically you could then seal a whole village into a person. But if you stick only to Naruto and Sasuke, I think the person you quoted already said most of it. It would depend on how exactly Itachi sealed Sasuke. I mean, technically you could also ask why he didn't seal Naruto into Sasuke, given that Sasuke supposedly can control Kurama.
I think the question of what their combined power would be is interesting though. Can't really answer it though, probably a mix of Sharingan and Sage mode like it says in the quote.