Here’s what half & half manga is about:

Nagakawa Shinichi and Sanada Yuuki both get involved in an accident and die together, but they are given a chance to live again. A voice lets them survive for seven more days. However, that after the time limit, one of two must die again.
Also, their lives will be shared, including emotions (guilt) and physical sensations (pain). They are told not to leave the other’s side, else both of them die. Now, with each of them wanting to save his/her own life, how will the next seven days of non-separation be like? Will they get along?