The foundation for reciting the Shema at two different times — evening and morning — is laid by Netivot Olam, Netiv Ha'Avodah 7:1, which explains that nighttime and daytime each express a distinct dimension of Divine unity: at night, when darkness signals the absence inherent in all created things, one proclaims that God alone endures; in the morning, when all existence is present and manifest, one proclaims that everything flows from Him — so each time-period demands its own affirmation of His oneness.
Ein Aya, Berakhot 1:1 builds on this by distinguishing the inner character of each recitation: the evening Shema, corresponding to exile, is directed inward — fortifying oneself before the waves that wash over Israel — while the morning Shema, corresponding to redemption and the rising of Israel's light, carries an outward, universal dimension, that all nations will ultimately recognize God's sovereignty.
As for the internal structure of the first paragraph itself — why 'Shema Yisrael' and 'Ve'ahavta' are treated as two distinct moments rather than one continuous flow — Mishnah Berakhot 2:2 records that R. Yehoshua ben Korcha explains the ordering as first accepting the yoke of Heaven's kingship and only afterward accepting the yoke of the commandments, establishing that the two are sequential obligations, not a single act.
Shem MiShmuel, Shoftim 6:5 sharpens this further: the people of Jericho did not pause between the verse of unity and 'Ve'ahavta' in order to show that the two are inseparable, while the Sages held the opposite — that a pause is precisely what is needed to demonstrate that intellect and heart are two genuinely distinct faculties, and that achieving alignment in one does not automatically bring the other along, for a person may understand with his mind yet have his heart drawn to lesser things, or the reverse.