Both dramas are set during the Japanese occupation.
Both dramas tell the story of good friends who become enemies due to different backgrounds.
Both male leads work under the Japanese government as undercovers.
Both male leads have multiple identity.
In both dramas, first and second male lead have the same love interest.

In both dramas, the female lead enters the world of the novel that she is currently reading.
The main leads are enemies with each other at first, but they fall in love eventually.

The two story concepts have one thing in common, a reversed timeline.
The difference is that in My Tomorrow, Your Yesterday the female lead is the one who has the timeline reversed.
While in An Ancient Love Song it is the male lead who experiences it.

Youth? Check.
Journey of discovery? Check.
Friendship? Check.
Great chemistry between leads? Check.
Natural acting? Check.
Friends (that's basically enemies) to Lovers tropes? Check.
The story maybe not exactly the same, but you will find similar vibes in both