Historical Rules · 14 June 2019
Rule 701.26b
Official source text from the 14 June 2019 Rules release
In a Two-Headed Giant game, poison counters are shared by the team. If more than one player on a team is chosen this way, only one of those players can be given an additional poison counter. The player who proliferates chooses which player that is. See rule 810, “Two-Headed Giant Variant.”