2 Become 1 is a pop ballad, written in the key of E♭minor, it is set in the time signature of common time and moves at a slow tempo of 72 beats per minute. The song is constructed in a verse-chorus form, and its instrumentation comes from a guitar, an electronic keyboard, and string instruments. The song opens with an instrumental introduction, with a chord progression of E♭m add9–D♭/F–G♭–A♭m7 sus4, that is also used during the first part of the verses. The last two lines of each verse changes the progression to C♭–B♭m7–A♭m7–D♭11, and changes again during each chorus to G♭–D♭–C♭–D♭. It closes with a string outro that uses the chord progression F♭–G♭–B♭♭–C♭, which is arranged by Scottish composer Craig Armstrong.