The Darkness in Me (Soul Eater)

Shinigami twins are extremely rare and are almost never born. So when Shinigami-Sama or Lord Death and his wife have twins that defiantly comes at a surprise for the death god. But since Shinigami's wife is a witch, that may have a negative effect on the death twins.

Death the Kid and Thana Death are always together. Thana's Lines of Sanzu are on the right side of her head and so when she stands beside Kid they are completely symmetrical. When they are apart they are filthy asymmetrical garbage. But that isn't all that is good about their Sanzu lines.

Twin Shinigami can match each other's wavelengths together even though they aren't meister and weapon and that causes their Lines of Sanzu to connect and the power of six Sanzu Lines is activated, making them even more powerful than a regular Shinigami whose lines are connected. So Artemis, Shinigami-Sama's wife, tries to use that power for her own gain to awaken the Kishin Asura, her oldest son, by turning Kid mad. Thana's love for her brother may not be enough to pull Kid back from the madness that is slowly consuming him.

"It won't be long now until Asura is finally freed. The world would be plunged into complete chaos."

Soul x OC
Kid x Maka

(Horrible summary I know, but the actual story is better)

Changes from the anime:

☆ Lord Death didn't create Kid from his soul but Kid was actually born from a mother
☆ Connecting the lines of Sanzu does not kill a parent Shinigami but turns them human
☆ Kid has a twin sister and they can both connect their Sanzu lines through soul resonance, but it is only temporally

Things that are still the same:

☆ Kid still has OCD (so does his twin sister)
☆Kid, Black☆Star and Soul still get into a fight on Kid's first day at school but it has Kid's twin joining the fight