Recommendations

Both are comedies with a great over the top male leads who are quite devious and hiliarious. If you enjoyed the performance of either one, you'll most likely quite appreciate the other.

Both series have a similar style (plus, the working title for M was so close to TEN that I thought it would be a spinoff at first), both series have a similar atmosphere while TEN is actually about murder cases compared to those cases about missing people.
Also the director Lee Seung-young was involved in the creation of both series, and both are OCN Original series (OCN is a channel with lots of experience when it comes to crime series)

Only based on two eps of FFI I got the picture of two persons trapped in one body which is similar to Who Are You? And one of the sides of one person is mean and the other side is nice.

In both dramas illness does not deter a person from pursuing love.
Both are touching and sweet.

Precious Time is my favorite J romance. Both show the devotion towards an ill person. Precious Time is more realistic and has a surprisingly fullfilling ending. Although the topic seems like a turnoff, it is upbeat and beautiful.

Same drama make by TVB. But so much more better. The main character is a lot more good looking!

Same drama made by TVB. It's the best! Xiao Long Nu in the version is the prettiest and most appropriately casted.

Similar plot, characters, story line, and era. But better looking actors!

Both dramas are intense, with fast moving plot filled with action as they both fight the time limit given to solve the mystery and defeat their opponent. You won't regret watching them!

Melodramatic
Another heartbreaking love story
A very sad love story
All about true love

Melodramatic
Another heartbreaking love story
A happy ending
All about true love

Melodramatic
Another heartbreaking love story
A happy ending
A drama that makes you believe in true love

Both males bring a female home in hopes of keeping their families quiet.

The feel to the movies are soo much similar! Both movies are soo lovable!!

Both are about being part of a modern royal family, about living in the palace and about royal guards. Both have romance, comedy, action and politics.
In both dramas the girl is choosen to marry a prince from another country. In King2Hearts it's a marriage between North and South Korea. In Lah Ruk Sut Kob Fah a girl from Thailand is about to marry the prince of Raya.