Chasimoh Tovoh

It has to do with the question if on RH there is Kesivoh and Chasimoh (as mentioned in Maseches Rosh Hashono 16b), or if the Kesivoh is at RH and the Chasimoh is at YOK (as mentioned in Rosh Hashono 16a, and as we say in Unesaneh Tokef). If the Kesivoh is at RH, we wish at RH לשנה טובה תכתב, and after that we wish חתימה טובה. And that is our Minhag. But if the Kesivoh and Chasimoh is at RH, one wishes at RH תכתב ותחתם לאלתר לחיים טובים, and after that one wishes גמר חתימה טובה, since the Chasimoh was already, and only the G'mar is missing.