The only thing I can say is that they might have troubles doing so because they're too connected. Could Rakan pass Illaoi's test if Xayah died? Would he even have the will to live? Same goes for her..
I think part of these two finding a purpose for their lives was/is tied into this relationship. They have goals they are chasing, they have love, they are doing what they think they should be doing with their lives. They are in motion.
To stay alive for one another would be more than enough will to get them past the test. If one was already dead, though, the surviving member would probably be a lot less likely to pass the test, especially Rakan. Rakan's only purpose in life is to be with Xayah, if she died, he'd be useless. Xayah has a goal of reclaiming the world or something, so she would still have something to fight for.
