Well, it comes down to some specific wording. Some cards, like Angelic Skirmisher, trigger “at the beginning of each combat.” An effect like this only triggers once per turn, even if there are two players sharing that phase/step, since it only cares that “combat is beginning.” (Also note that the “you” in Angelic Skirmisher’s text really means “you” and will only give a bonus to the creatures controlled by that one player on the team.)
The wording on the Sentinel is slightly (but importantly!) different, because it triggers “at the beginning of combat on each opponent’s turn.” This means the trigger condition is “an opponent is starting combat on their turn,” and in 2HG that happens twice per combat, since you have two opponents and they are each beginning combat on their turn.
The end result is that you get to tap down two creatures (one controlled by each opponent) every time the opposing team starts combat on their turn. Pretty strong stuff!
Today’s Rules Tip written by Josh Stansfield