How to have more than 1 unique demons in Devil Survivor 2? After starting new game+ do I still have all the demon fusion or only the ones that I unlocked. Can all the fusions be completed without the required demons that I got in the first game through.
You cannot have more than 1 of each unique demon in your party at a time, that's the whole purpose.
As for new game+, you can find that information in this other question.
And no, you need multiple playthroughs to unlock everything as there are mutually exclusive branches in the story where unique demons and abilities show up for unlocks.