Rule 805.3 · Shared Team Turns Option
The methods described in rule 103.1 are used to determine which team will take the first turn. The team determined this way is the starting team.
Subrules
The process for handling mulligans is altered accordingly. First, each player on the starting team, in whatever order that team likes, 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. Teammates may consult while choosing which cards, if any, to put on the bottom of their library. A player may take a mulligan even after a teammate has decided to keep their opening hand. See rule 103.5.
The process for handling cards that allow a player to begin the game with them on the battlefield is altered accordingly. First, each player on the starting team, in whatever order that team likes, may put any or all such cards onto the battlefield from that player’s opening hand. Teammates may consult while making their decisions. Then each player on each other team in turn order does the same.