So I had an interesting Judge call today at a two-headed giant pre-release event. The question was:

if Dragons was chosen as the anchor word, does the controlling player get to tap one creature for each opponent at the start of combat.

Citadel Siege
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.

The problem is that in this specific multi-player format both opponents have shared turns, and all wording on the card is singular, thus insinuating a single tap per turn. After reading the card and looking and the board state I determined the reasoning for the call as it would decide the match. Thus I consulted the release notes for the specific card, and saw that it did not have any release specific notes. Time was getting short and I needed to make a call. Ultimately I ruled that they did. As I was the only judge in attendance the ruling stood.

After the call I consulted the comprehensive rules, but was unable to find any relevant rules concerning the call. I am interested to know how others would have handled this situation, and if I handled it correctly.

In a Two-Headed Giant game, Citadel Siege will trigger separately for each opposing head, and will tap down one creature from each, for a total of two creatures.

The reason for this is that Opposing Head A and Opposing Head B are each an opponent. The beginning of combat on the opposing team's turn is therefore both “the beginning of combat on Head A's turn” and “the beginning of combat on Head B's turn”, triggering the ability twice.