בברכה
שמעון דוד קורץ
It is worth noting that the Teymani tradition is slightly different in that
the two rows aren't justified at each end, but ragged (see photo from the
Yemenite tikkun I have).
To a large extent the fact that Ha'azinu is a poem and is spaced in two rows
is rather taken as read. Masechet Sofrim 1:9 explains that one musn't write
it in the form of Shirat Hayam and vice versa - so accepts that it has a
special form and later in 12:8 talks about R. Jose b Abin's views of a line
above and below and 12:9 the beginning words of the lines - so again takes
it for granted that the shape is known. This is a very early source.
The Mishneh Torah Hilchot Sefer Torah 7:9 talks about the six lines above
and fine lines below and chapter 8 goes into detail on the form of the poem.
Keset Hasofer 16:2. [Concerning] the poem Ha-aziynu [Lit. give ear] we also
leave before it and after it a completely empty line. And we write the poem
in seventy lines, each line divided into two in the measure of a closed
section.
Mishnat hasofer notes that 'they must be equal at the start and end as some
say that not having this prevents [validity]' which would suggest he would
invalidate the Teymani tikkun.
I hope that this helps.
Shabbat Shalom,
Mordechai