In terms of story they are slightly different but are similar as to the relationship dynamics between the main couples and the psychological vibes. Both can be classified as romance that at some points have unsettling events happen between the couples.

Both movies depict the love stories of a couple where one person has dementia and their significant others don't.

They are super similar with the main difference being in one movie the FL has dementia and in the other the ML has dementia.

Similar vibes but in Og My Drama Lover FL is a character in her own written story whereas Meet by Window FL is a character in someone else's story (and able to come and go)

I find both movies VERY similar in terms of theme. They both involve the FLs traveling to another universe which happens to be inside of a book.

Both involve demons and shamanism. While "possessed" mainly focuses on the relationship of the male lead and female lead "Coming" has very little romance between one of the male and female leads.

While "Coming" doesn't involve priests both movies focus on exorcism and possession.

Similar themes of a man and his family being possessed by powerful demons and exorcism.