When it comes to the parsha relevant for vehaya im shamoa it is clear that it needs to be stuma since that's how it's written in the torah.
MB brings the well known Taz who came up with a "solution" of writing a parsha stuma according to everyone. He says to leave less than 9 otiyot ketanot at the end of shma, do the same at the beginning of vehaya on the next line and it's as if by combining the two lines we would have a parsha stuma for all de'ot (which can only happens when there's a space of 9 letters in the middle of the same line).
MB further writes that a normal parsha stuma for all de'ot isn't possible simply because each parsha needs to be written in a separate column, therefore shma and vehaya can't be in the same line.
MB brings that Rema MiPanu, GR"A and Pri Megadim agree with the Taz and in Biur Halacha he brings that GRa"Z holds the Taz's solution to be pasul and Ma'amar Mordechai is also doubtful.
Lema'ase I've learnt that one should follow the minhag of his family in choosing between Rambam or Taz.
My questions are:
1) For many people that don't know their family minhag, ba'alei teshuva, geirim etc I understand that either they follow what their Rav decides or if they don't have one single Rav they can choose which way to go. I'm interested in what other sofrim would say if they were given the choice.
On the one hand, to be lechatchila (according to most opinions) you need 9 otiyiot gedolot, which is only possible according to Rambam. But if you go like Rambam it isn't aliba dekulei alma.
On the other hand going like the Taz a) not everyone holds that the tzeiruf concept (combining the two lines) is acceptable, in which case according to them it's bediavad at best, pasul gamur at worst and b) even if it is, since the parsha is 9 otiyot ketanot it's not really lechatchila to most opinions (definitely not mehudar).
2) The fact that shma and vehaya must be written in two separate columns is brought in hilchot tefillin. Regarding mezuza there are many de'ot how to do this parsha. Pitchei Shearim brings many opinions, including that Mahari Abuhav (I was once told that this reference is a mistake and should be someone else, but I can't remember who) holds to write the end of shma and vehaya on the same line to make a true parsha stuma aliba dekulei alma. Pitchei Shearim then says that this is "תמוה לכו"ע". Also when I asked R Mordechai Friedlander of Mishmeret Stam in Jerusalem he said this can't be done because the need to be written on separate lines. My question is why? I haven't found a source as to why in a mezuza they must be on two separate lines. Possible answers I've heard over the years are:
a) Since in the torah they aren't mamash stuma next to each other, therefore they must be on separate lines.
b) Since according to kabbalistic source the shadai must be written corresponding to vehaya and must be visible when rolling up the mezuza. Therefore by writing them on the same line shadai will be at the other side of the mezuza and will be covered up when rolling. (this to me can be refuted by simply, however unconventional, rolling the mezuza in such as way that it has a very large diameter and can still see the shadai. Albeit a case will be hard to come by).
c) Since in tefillin they are two separate columns so to in mezuza they must be two separate lines.
But I've never found a definitive source. Thoughts?