Income from a song or piece of music can be split in any way AS LONG AS THEIR IS A WRITTEN AGREEMENT in place between the parties. If there is no agreement, then income would be split equally between the parties.
Traditionally, music represents 50% of a song and lyrics 50%: with the increased importance of production over the past 20-25 years - not to mention the fact that vocalists often write the vocal melody but not chords - it is not unusual for melody to be 1/3rd, lyrics to be 1/3rd and chords/arrangement to be 1/3rd. Anything is possible but the default arrangement is 50/50.
Income from a song or piece of music can be split in any way AS LONG AS THEIR IS A WRITTEN AGREEMENT in place between the parties. If there is no agreement, then income would be split equally between the parties.