Fake and Real Young Masters: My Empress Is My Mortal Enemy

Fake and true young masters + gender swap + enemies to allies + mutual devotion. In the modern world, he was the Lu family's true young master, while the other was their fake one. A rigged car race ended with both of them dead.

When they opened their eyes again, one had become Emperor Ji Yun, and the other, Empress Liang Yu.

Enemies turned into spouses, and a man turned into an empress. One built political alliances by winning over the concubines in the harem, while the other used the emperor's art of rule to counter each move at court.

But when their battle reached its breaking point, they discovered that the people they truly ought to hate had never been each other.