These two drama:
—deal with the entertainment industry
—have eccentric accomplished male leads who deal with certain health conditions
—the female leads have both a notorious reputation
—the couple goes through various difficulties to be together

The two drama are similar in:
—Eccentric chaebol male lead
—gutsy female lead who is working in the entertainment business
—a "paranormal" event brings them closer
— the secretary/manager are doing an excellent job of being there and always rooting for the main couple
—the secondary couples' stories are also prominent

These two dramas are similar when it comes to the main leads being wrongfully accused of deeds they didn't commit yet took the blame/fall for. How one wrong accusation ruined their life. Finding out the truth over that event is also a similar theme in both drama.

Borh dramas deal with time travel and the attempts of the main lead to change the past/future. The decisions of the main lead are of great importance for the time-related plot and how small changes alter a person's course in life.

Both deal with time as their central theme and the struggle to find out the truth over a past event and how small changes can alter one's life in unexpected ways. Despite the fact that Reunited Worlds focuses on the Past, Tomorrow With You focuses on the future, the feeling they emit is similar