[@Lugubrious]

Disgaea Netherworlds are pretty much a multiverse of their own. Each game is canon, but really has no effect on the next aside from the latest entries always allowing you to recruit the old casts in the post-game stories. So Mao defeats and recruits Laharl in Disgaea 3, and then Laharl pretty much does the same to him in another game. 

Essentially what happens in Disgaea 3 is that Laharl comes in pretending to be Mao's true father (Despite their similar age) So he can be more relevant in the game. This escalates quickly to Laharl trying to pummel Mao for the main character spot instead, but I'm not quite sure if post-game stuff is canon or not so it's up in the air if they know each other. Regardless, they'd easily be able to tell that they're both Overlords, there's just a bit of a sixth sense about that in Disgaea. So overall it'd be a fun interaction, I think.

[@Dark Cloud]

Laharl's father was simply the king of [i]his[/i] Netherworld, not many of them.