The Posas Tribulation is also the Tribulation of Life and Death. It can affect a person's life and even change his or her life. If you survive it safely, you can become an immortal; if you don't survive it, your bones will be shattered and ashes will be thrown away.
All living beings, from gods to demons, all have their own tribulations.
If two people suffer the same calamity as one, only one of the three will survive, unless he exhausts his cultivation and falls into Yama forever, which is irreversible.
What would happen if Sun Wukong and Murong Bai were the same person?