RB1's true end doesn't make a sequel completely impossible like the original game's true end did, but Arfoire's different roles in the two games arguably make it impossible to really say that RB1 and RB2 are in the same universe.
That said, Neptunia is not a game that has ever really given enough of a fuck about the fourth wall for that sort of thing to be worth worrying about. RB1 makes a joke referencing a bad end from mk2/RB2, for fuck's sake.
And as far as which one should be played first, I'd still recommend RB1, if only because while RB1 may not be "canon" RB2 still assumes you're familiar with Nep, Noire, Blanc, and Vert. The main four goddesses and the chemistry between them is the core of the appeal of the whole franchise, and if you start with RB2 you won't really see that in action for a while, since it's centered around the goddesses' little sisters, who don't have anywhere near the level of chemistry that the main four do, which is why it's generally considered to be the weakest one.
