Fucked my brain !

It's similar to the two doors riddle scene in Labyrinth.

Well, actually, not brain fucked. But got this type of problems many times so...

Got four lines down in the explanation before my brain just decided to figure it out.

a is knight = paradox
(If b is a knight, they're a knave and if they're a knave, they're a knight)
a is knave = fine
(a is knave and b is knight = possible?;
both knaves = all fine)

Edit: reread the question and I misread the question 😋