Both dramas deal with powerful plotting women and men that are changing sides. In both dramas a murderer plays an important role and involves nearly all the people.
Although the K2 has more action to offer for now, the suspense and the mystery and plotting between the characters is a lot similar.

Both deal with police officers who talk with people who are involved in a crime/or about to commit a crime.
Both have a main pair that tries to solve one big case that connects them via losses

Both dramas have a similar with how they portray those "time skips". Both deal with a detective who has a connection to the future. In Signal via a transponder, with Tunnel via a Tunnel. Both are crime/investigation kdramas.
Both male leads have had once a love interest who was played by the same actress.

Both movies focus one a criminal who reaches out via a live broadcast. Both movies have a live debate with the culprit. However Terror Live is more intense.

Both movies are very when it comes to characters, and themes. Both movies have a catastrophic event. That causes terror and chaos.

Both heartbreaking romance stories. Both have an amazing storyline. Both deal with a terminal illness.

Both movies focus on a love story, both have a male character that has a dark past, both have a female lead that has an handicap. Both couples have to overcome tragedy

Both are excellent crime/investigation jdramas and both have Kimura Fumino as female lead.
Siren focuses more on the romance, the case is good and twisted. Ishi no Mayu has no romance but darker even more twisted cases.
both recommended

In both kdramas the theme is used: Money is power.
Both deal with corruption and murder. Somehow Signal ended for me where the story of The K2 might start.
2 of the best kdramas that got released in 2016
definitely a must watch!