I want to ask, how New cards, which would be triggered by opponent's step/ phase, works in 2HG. Such as or .

How many of times would the cards be triggered in opponent's relative step.
and How they works?

There is an argue in my community that
1) Some believes it would be triggered 2 times
2) Some believes it would be triggered 1 times, but affect the whole team's players (like. Archfiend of Depravity)

For Citadel Siege
3) Some believes it would be triggered 1 times, affect the whole team's players but look for ONE target only. (target creature controlled by that team's players)

Thanks



Card Ref:
Citadel Siege 2WW (4) Enchantment
As Citadel Siege enters the battlefield, choose Khans or Dragons.
• Khans — At the beginning of combat on your turn, put two +1/+1 counters on target creature you control.
• Dragons — At the beginning of combat on each opponent's turn, tap target creature that player controls.

Archfiend of Depravity 3BB (5) Creature - Demon 5/4
Flying
At the beginning of each opponent's end step, that player chooses up to two creatures he or she controls, then sacrifices the rest.

This question was already answered, here.