Historical Rules · 29 September 2017
Rule 103.4d
Official source text from the 29 September 2017 Rules release
In a multiplayer game using the shared team turns option, first each player on the starting team declares whether or not he or she 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 his or her teammate has decided to keep his or her opening hand.