か～ないかのうちに

か～ないかのうちに says the second event came so fast that the first had barely happened: "hardly had… when", "the instant". The same verb appears twice – the dictionary form with か, then the ない-form with か – and のうちに follows.

The first verb can also be in the た-form: はなし話しおわった終わったかおわらない終わらないかのうちに. Either way, the second verb is the ない-form of the same verb, not a different one.

Only for what happens

The pattern reports two events as facts, so the second clause states what happened, or what always happens. It can't be a request, an order or something you intend to do.

• × 家に着くか着かないかのうちに、電話してください → 家に着いたら、すぐ電話してください

か～ないかのうちに or たとたん

たとたん says the first action was over and something followed at once, usually as a surprise. か～ないかのうちに puts the second event before the first was even finished, or at the same moment.

The pattern is a little formal and turns up mostly in written narrative. In conversation you would say たとたん or てすぐ.