The Spirit of the Ruler (2017)

Both series have romance between a warm, level-headed doctor and a cool military man. There also have bandits and military missions.

they are the same series except the main character role is shifted. There are times when they have combined missions or they crossover to each other's plot.

They are similar because,
Firstly, both are story of a person who's work is serving his country.
Secondly, male lead fall in love and that girl helped him in his work.

Both ML are soldiers and got the job to protect the FL.
In Both there is a misunderstanding between FL and ML.
In one FL is a princess in the other a physician.
Both are series. One about twin brothers. The other about soldiers friendship.
I like the acting and would say everyone who like military should watch both.