By the Beis Horah today R' Shammai Gross had 3 people asking the same shailo. They are all making hachnosos Sefer Torah on Shavuos itself and the Sefer Torah will not be finished by then. The Sofer however has the sefer that is missing already written from another Sefer Torah that they want to now put it all together in order to make the chachnasos Sefer Torah on Shavuos and then later take it apart when the Sofer finishes whatever Sefer that is missing. R' Shammai l'maseh said it's muter to do as long as a tanai is made when they are tofer everything together. If anyone has anything to add on this or any thoughts, please share.