from what I'VE scene P2: EP is more of a direct sequel to P1 than P2: IS which is also one of the reasons I think EP got a stateside release over IS. Both P2s rub me wrong so whatever.
Well Persona 2 has the Kuzunoha Detective Agency, so you could argue that Devil Summoner and Persona take place in the same world, though I think that was more of an Easter Egg for fans than anything important.
I THINK it's the same world since the reference is pretty pervasive in uh IS I think. It's just some really obfuscated way. SMT If... is also in there. I never played the first two devil summoner games or SMT If so I couldn't get the details without horribly butchering the hell out of them though.
Also the character design for Raidou Kuzunoha was invented sometime in the late 90s, which is odd.