The basic issue in Gitin is this: One says to two people to write a get and give it to a messenger. The Gemara asks, "Does he mean to write it once only, or to write it until it gets done?" This is left as a doubt. The Rambam says the question is referring to a case when the get was found to be null. To Rashi, the issue is if the get was lost, but if found null obviously one could write another"
To Rav Shach the argument depends on the question if writing the get [divorce] requires being a messenger or simply command of the husband.
What Rav Shach means is: The difference is this: If writing the get [divorce] requires to be a messenger from the husband, then we can understand that after he has written a get [divorce] and it got lot it might be a doubt if he has fulfilled his mission. This might be the doubt in the Gemara on page 63 maybe the messenger-ship was filled since he wrote the get as commanded. This is how Rav Shach explains Rashi that holds the doubt of the gemara is if it was lost. But if simply found to be null, then of course he can write another. And this part of the answer of Rav Shach makes sense.
However even if we say that the Rambam holds the writing does not require messenger-ship rather only a command of the husband, still I can see that there is the exact same doubt about how far his command extends? To write it just once, or at least once. But to say the argument between Rashi and the Rambam does not seem to depend on the question of if the get requires "messenger-ship or only a command."
I later was on my way to the sea and it occurred to me that we can understand Rav Shach in this way. Let's think about the difference between making a messenger and commanding. Making a messenger to give a divorce doc could not apply in any way to an invalid get. But commanding to write a get can apply to an invalid get. So we can understand the Rambam that telling two people to write a get can be similar to when the husband himself writes a get. It can be found to be invalid. And if so he can write another. But if his command to write a get can apply twice-that is the question of the gemara. So in short Rashi holds telling two to write a get is from messenger-ship and thus writing twice can only be a doubt if the get was lost. But there certainly can be no doubts if the get was found to be invalid. He certainly did not appoint the two to write an invalid get. So they can write another.
But to the Rambam who holds it is merely a command and thus there can e a doubt if it was found to be invalid if they can write another.
