Historical Rules · 26 August 2016
Rule 118.4a
Official source text from the 26 August 2016 Rules release
If a cost or effect allows a player to pay an amount of life greater than 0 in a Two-Headed Giant game, the player may do so only if his or her team’s life total is greater than or equal to the total amount of life both team members are paying for that cost or effect. If a player pays life, the payment is subtracted from his or her team’s life total. (Players can always pay 0 life.)