Responses
below might be considered a spoiler
but that explanation makes me sad. I always hoped Siwon somehow lived on inside TaeMin. The Siwon story is beyond sad.
beta
I thought that Siwon might have loved Siwoon from afar, so when Siwon died, wandering the hospital in his astral body, he came upon Siwoon/WooJin grieving for Tae Min who had just died. Siwon invited Tae Min to take his body that he no longer occupied.