901.12 A Two-Headed Giant Planechase game uses all the rules for the Two-Headed Giant multiplayer variant and all the rules for the Planechase casual variant, with the following additions.
901.12b
The planar controller is normally the primary player of the active team. However, if the current planar controller’s team would leave the game, instead the primary player of the next team in turn order that wouldn’t leave the game becomes the planar controller, then the old planar controller’s team leaves the game. The new planar controller retains that designation until they leave the game or a different team becomes the active team, whichever comes first.
901.12c
Even though the face-up plane or phenomenon is controlled by just one player, any ability of that plane or phenomenon that refers to “you” applies to both members of the planar controller’s team.
901.12d
Since each member of the active team is an active player, each of them may roll the planar die. Each player’s cost to roll the planar die is based on the number of times that particular player has already rolled the planar die that turn.