Ah, I see my misunderstanding now, sorry.
Of course, you can install EFIDroid and a primary OS, and that may indeed work just fine. But that’s not what you need EFIDroid for.
EFIDroid (as it says in the topic title and in the first post) is a multiboot solution.
And as soon as you add another OS to EFIDroid apart from the primary one, booting that additional OS doesn’t work currently on the Fairphone 2, this was confirmed by several users in this topic.
So, to clarify, EFIDroid currently doesn’t work as a multiboot solution on the Fairphone 2, so at the moment it doesn’t do the very job it is there for to do.