While I do not know where you originally saw this I found references to it by googlin
machlokes second day creation
Rabbi Frand on Bereishis goes into this subject.
On the second day, G-d divided the upper and the lower waters. This
was not a case of good water and bad water; of True water and False
water. This was a case of making a division between two equally valid
components. Regarding such division we do not say, "It was good". This
was an unfortunate division. A division was necessary, but there is no
'ki tov' on that day because conceptually there is no reason to have
machlokes between 'water' and 'water'.
But the first day was different. On the first day, the division was
between Light and Dark (Or v'Choshech). By analogy, this represents
separation between Truth and Falsehood, between the forces of Good and
the forces of Evil. There we must divide. We must delineate. We must
say this is Light and this is Dark; This is True and this is False.
This is a machlokes, but it is a machlokes that warrants a 'Ki Tov'.
It is a necessary machlokes -- a division that must be made.
A reference to Rashi in Parshas Korach
Unlike the other days of creation, the Torah does not conclude its
description of the second day with the words 'Ki Tov'. On this day,
G-d completes His work without seeing the good. Chazal explain that
Gehinnom and Machlokes (dispute) were both created on Yom Sheni, as
indicated by the splitting of the heavenly and earthly waters. This
day remains unfinished, its Tov not evident until Yom Shlishi.
Each of the daily songs, the Psalms recited at the close of morning
prayers, reflect the particular aspect of creation revealed on that
day. Hence, on Yom Rishon, we proclaim that 'the world is God's and
everything in it'.
On Yom Sheni we repeat the song of the B'nai Korach, the beauty of
Jerusalem and the palace of the King.
Who were the B'nai Korach and why do they sing on Yom Sheni?
"And the sons of Korach did not die - They were in the original plan.
At the time of the Machlokes, they had thoughts of Tshuva, therefore
an elevated place in Gehinnom was established for them, and there they
dwelled." (Rashi, Bamidbar, 26,11)
For Heaven's Sake!
The Ohr Gedaliyahu explains that on the first day of Creation, which
the Torah calls “the Day of One,” the revelation of Hashem in the
physical world was perfectly clear. Multiplicity and disparity were
not yet part of the physical world in a way that would hide Hashem’s
Oneness. On the second day, the concept of diversity and multiplicity
became a reality in the world (the firmament separated between the
waters). It is this multiplicity that is the essence of physical
reality, which Hashem uses to hide the clear manifestation of His
Oneness, thus setting the stage for the operation of human free will.
The second day of Creation represents a mixed bag of sorts: on the one
hand, Hashem’s purpose is that His Unity should be hidden by physical
reality only to be revealed by the efforts of Jews making free-will
choices; on the other hand, the multiplicity actually creates the
possibility for the very erroneous perception that Hashem, ch”v, does
not exist! This latter point explains why the verse כי טוב (“It was
good”) does not appear on the second day; how could it be remotely
good that there could be even a thought that Hashem does not exist,