Seems to depend where you look, it is mixed between mutual strike or mutual killing...http://www.koryu.com/library/gbuyens2.html
"Timing, distance, and appreciation of the opportunity all lead to the ability to take initiative, captured in the word sen. Traditionally, there are three major combative initiatives described: go no sen, which is the initiative after an enemy has already launched his own attack; sen no sen, where one prepares to meet an attack (also called saki), and sen sen no sen, where one anticipates an attack (also called sen-no-saki, kakari no sen). All these initiatives can determine the outcome of an encounter, which could be: katsu (victory); make (defeat), or ai-uchi (mutual killing).
In modern budo, ai-uchi is an acceptable outcome for a confrontation. During kendo or karate competition for example, an ai-uchi or mutual execution of an effective technique is ignored in the scoring and the fight is continued. In older budo, this is not the case, since ai-uchi would lead to mutual death, an unacceptable outcome. A samurai would never attempt ai-uchi except when faced with an unavoidable confrontation with an opponent with known superior fighting skills. In that case ai-uchi would be a possible tactic and he might be willing to engage a technique to kill his opponent, knowing that there is a risk to be killed as well. This concept of risk-taking is very important. Certainly with bladed weapons, being touched by the opponent while attacking him is far more dangerous than risking a blow to a non-vital part of the body from an unarmed opponent or from a training partner with a non-cutting weapon, who is controlling his movement. In fact, ai-uchi can be written using two different characters (kanji) for uchi: the first translates as beat, attack, defeat, conquer; the second is closely related but slightly different and translates as attack, defeat, destroy, conquer."