Historical Rules · 1 June 2020
Rule 103.4d
Official source text from the 1 June 2020 Rules release
In a multiplayer game using the shared team turns option, first each player on the starting team declares whether that player will take a mulligan, then the players on each other team in turn order do the same. Teammates may consult while making their decisions. Then all mulligans are taken at the same time. A player may take a mulligan even after a teammate has decided to keep their opening hand.