Magmatic Force

• WWizards of the Coast2011-09-22
  Official Ruling
  In a Two-Headed Giant game, this ability triggers once at the beginning of each team’s upkeep.
• WWizards of the Coast2011-09-22
  Official Ruling
  The controller of Magmatic Force when its ability triggers always chooses the target, even when the ability triggers during another player’s upkeep.

Rulings via Scryfall