One of the main leads is riding a parachute as a hobby to escape family problems. The wind directed them and have them crash landed to an environment very unfamiliar to them. In this strange new environment is where our main lead met his/her future lover and where he/she develops a close relationship with not only his/her lover but the whole townsfolk living in that new environment. One half of the lead couple also each happened to be engaged to marry another person his family members wants him to marry which he did not love, main difference being that the one engaged in CLOY is the person who lives in the new environment while the one engaged in Forevermore is the guy who crash landed to the environment. The political situation of the two Koreas in CLOY is also another obvious difference.
It's also interesting that the actors playing the couples in both dramas are real life couples and that these two dramas are not the first time each of the two couples acted together. Each couple did a movie together (Forevermore couple - She's the One 2013 movie; CLOY couple - The Negotiation 2018 movie) where they did not play couples before these two dramas. Before the two movies, both couples also did projects where one-half of the couple is an important character in the project while the other one was a cameo in that project (Forevermore couple - Xander actor did a cameo in Must Be Love 2013 movie that featured Agnes actress as the third wheel; CLOY couple - Yoon Se Ri actress did a cameo in Secret Garden 2010 drama that featured Captain Ri actor as the main male character).

Both sageuks have important events that happened outside Joseon. There is also a non-Korean character in a high position in love with at one of the main leads.

=> rich, stylish and arrogant female lead falls in love with a non-human being who can live forever.

=> both are Kim Yoo Jung dramas
=> both Kim Yoo Jung characters fell in love with a man who has "demon" qualities

=> time travelling
=> the time traveller from Joseon lost his/her lover in his/her life in joseon
=> time traveller is looking forward to find his/her love in the modern era
=> Bonus: Both Han Ji Min (female lead in Rooftop Prince) and Lee Se Young (female lead in The Story of Park's Marriage Contract) once played Concubine Uibin Seong (beloved concubine of King Jeongjo of Joseon) in Yi San (2007 drama) and The Red Sleeve respectively.

=> Both are set in the Qing invasion of Joseon
=> early parts of the story revolves around marriage but slowly turns to war and carnage
=> Both dramas depict how the Qing invasion really affected many people but young people who are at the stage of their lives to fall in love and get married have quite a focus.
=> There are important Qing characters.
=> Strong male and female characters (my fave actress Moon Chae Won got many awards for her depiction of the female lead in War of the Arrows => people should watch her sageuk works especially this, Princess Man, and Painter of the Wind).
PS: For War of the Arrows fans, the show being recommended is Part 2 of My Dearest, you should check part 1 first for the beginnings of the story.

=> Both are historical Kdramas partly inspired by Western love stories. If My Dearest is partly inspired by Gone with the Wind, (as described in wikipedia), The Princess' Man is partly inspired by Romeo and Juliet.
=> Earlier episodes contain fun and lighthearted scenes before political events changed everything
=> Female lead rides a swing in both kdramas, both scenes contributed to male lead falling in love with her
=> There are scenes in both dramas of couples trying to run away.
PS: For The Princess's Man fans, the show being recommended is Part 2 of My Dearest, you should check part 1 first for the beginnings of the story.

=> Both are classic melodrama romance sageuks relased within two years of each other (early 2020s).
=> Both Male leads are madly in love with the female lead, male lead is very consistent with his feelings and desire to pursue and be with female lead
=> Female lead tries her best to resist her love and feelings for the male lead
=> Male leads have male friend who is the closest we can consider for the female lead's rival of male lead's affections. Of course both male leads only have feelings for our female lead. (In Red Sleeve though, this male friend is more like just a loyal friend with no romantic feelings, but some fans do think he has sort of feelings for the male lead).
PS: For Red Sleeve fans, the show being recommended is Part 2 of My Dearest, you should check part 1 first for the beginnings of the story.

=> Both are classic melodrama romance sageuks relased within two years of each other (early 2020s).
=> Both Male leads are madly in love with the female lead, male lead is very consistent with his feelings and desire to pursue and be with female lead
=> Female lead tries her best to resist her love and feelings for the male lead
=> Male leads have male friend who is the closest we can consider for the female lead's rival of male lead's affections. Of course both male leads only have feelings for our female lead. (In Red Sleeve though, this male friend is more like just a loyal friend with no romantic feelings, but some fans do think he has sort of feelings for the male lead).
PS: For Red Sleeve fans, the show being recommended is Part 1 of My Dearest, you should check part 2 for the continuation of the story.

=> Both are kdramas of the same story: My Dearest. Each of the two shows recommended are two seasons of the same show.

Both sageuks have really good plots with the same writer. Both male and female lead are strong characters who have to overcome a lot. Both have good OSTs, Main difference is that there is less focus on romance on Rebel Thief than My Dearest and that the main couple in Rebel Thief are friends for long before becoming a couple. Rebel Thief also has a few fantastical elements not present in My Dearest.

Both sageuks have really good plots with the same writer. Both male and female lead are strong characters who have to overcome a lot. Both have good OSTs, Main difference is that there is less focus on romance on Rebel Thief than My Dearest and that the main couple in Rebel Thief are friends for long before becoming a couple. Rebel Thief also has a few fantastical elements not present in My Dearest.