Suicide pact
A suicide pact is a plan of two or more people to kill themselves. People who agree on such a plan may kill themselves together, or they may just plan to do it on a certain schedule together. When studying suicide, suicide pacts are an important concept. Suicide pacts have occurred through history.
Note that a suicide pact is different from mass suicide. In a mass suicide, many people commit suicide for the same reason. In a suicide pact, it might be that one person is suicidal because of romance, and the other is suicidal because of a disease, but they both wish to die.