Missed triggers in two-headed giant
If the primary player has moved forward to the draw step, then the trigger was missed. If the secondary player is the one that drew a card, then that player has drawn extra cards.

In either case, at regular REL, you should call a judge, explain what happened, and allow the judge to decide whether the trigger was missed or whether the game should be rolled back. If the game is rolled back, you would have to put a random card from your hand on top of your library, then do your upkeep stuff, and then continue as normal with a reminder to play more carefully. From Judging at Regular:


  If the identity of the card(s) were known to all players, return them to their proper location. Otherwise, determine how many extra cards have been drawn, take that many cards at random from the player’s hand and place them on top of the library. Don't shuffle the library after doing this!


In general, play penalties at regular REL are basically nonexistent, and mistakes should be fixed more often than punished.