It is pashut to me, that to glue a matlis after the start of the writing of the tefilin or mezuza, and then write on the matlis [csidran] is not accepted [it is pasul], since this is writing a mezuza [and tefilin have the same law] on 2 parchments, as explained in Yore Dea siman 288:4.
My question is, if he glued the matlis before the begining of the writing tefilin or mezuza, may he then write part of the parshiya or mezuza also on the matlis (obviously only complete letter/s with the hekef gvil as in a sefer tora).
See Pishei Teshuva YD 288:4 in the name of Shut Rabbi Akiva Eiger that permits gluing 2 parchments before the csiva and writing after they are combined, a mezuza. Seemingly the same should apply to a matlis that was glued before, but maybe matlis is worse than a complete piece of parchment?
See Mishnat Hasofer 27:5 that there is an argument, if a mezuza written on 2 parchments glued [or sowen] before the csiva, is kosher. so if a matlis is like 2 parchments we are into a machalokes!