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.
The lyrics focuses upon how the bonding of two lovers can become so strong that they practically become one entity, through the act of sexual intercourse. Apart from the sexual connotations, there is an aspirational undercurrent to the lyrics, and like many of their subsequent songs, desire is explicitly linked to ambition –"Free your mind of doubt and danger/Be for real don't be a stranger/We can achieve it/We can achieve it".
There are two different versions with different lyrics. After having a huge gay fanbase, the girls rewrote the second verse to not single out any of their fans. In the "album version", the second line of the second verse is "any deal that we endeavor/boys and girls feel good together". In the "single version", the same line is changed to "once again if we endeavour/love will bring us back together". The pitch of the second "take it or leave it" in the following line is also different. Victoria Beckham sings on the "single version", while Geri Halliwell sings on the "album version". On the music video and on stage they performed the "single version", both before and after Halliwell left the band. Though after Halliwell left the band, Victoria took over her vocals on chorus during live peformances. The "single version" is included on their greatest hits album.