Historical Rules · 1 February 2010
Rule 806.10b
Official source text from the 1 February 2010 Rules release
If an effect would set the life total of each player on a team to a number, the result is the sum of all the numbers. Example: In a Two-Headed Giant game, a player casts Biorhythm, which reads, "Each player's life total becomes the number of creatures he or she controls." If one member of a team that has 25 life controls three creatures and the other member controls four creatures, that team's life total becomes 7. The first player is considered to have lost 10 life (13 - 3), and the second player is considered to have lost 9 life (13 - 4), even though the team didn't lose a total of 19 life.