Both dramas have
1.Two male leads. One good with with sword, other in solving cases.
3. Chemistry between the two male leads
2. Solving cases happen in both.
4. Both moving towards solving the ultimate mystery/conspiracy?

-just my initial thoughts. Will update later

Leap Day is supposed to be both a sequel and prequel to 940920. A story with same cast that happens in a different space-time. The female lead from leap day ends up in the time-space dimension of 940920. Episode 10 of Leap day could be considered as the first episode of 940920. In order to avoid confusion, watch Leap day before 940920. And for those who are confused by the ending of Leap day, the answer is 940920.

Both ml saved the respective fl (though under different circumstances) while they were young.
Smart capable FLs involved in investigation with the MLs.

The legend of Dugu concerns with the fate of dugu sisters and follows history until Emperor Wen of Sui's Reign
Heroes in Sui ans Tang Dynasties is about the heroes who rose to prominence in the latter period of Sui's reign and helped establish the Tang Empire. To those who are looking for a continuation of the story.

Both concern with the training of a special taskforce to fight Terrorism. Kungfu Cop has a romance with happy Ending.

A selected few after initial screening gets training in military/airforce faces challenges and grows together as a team.