In the Two-Headed Giant format, you still only have to pay once per creature.

You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t attack unless the controller chooses to pay the cost (a cost of zero is not automatically paid, so the controller can choose to not pay).

The ability only applies to attackers declared after it resolves. It will not add costs to any attackers already announced.

War Tax : règles, fonctionnement et interactions (MTG)

War Tax is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: This turn, creatures can't attack unless their controller pays for each attacking creature they control.

How it works : War Tax

Original English text — Wizards of the Coast / Scryfall

#1February 1, 2007· Wizards of the Coast
In the Two-Headed Giant format, you still only have to pay once per creature.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t attack unless the controller chooses to pay the cost (a cost of zero is not automatically paid, so the controller can choose to not pay).
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
The ability only applies to attackers declared after it resolves. It will not add costs to any attackers already announced.
No other card is named in this ruling.