Updated March 18, 2011
Two-Headed Giant Constructed decks must contain a minimum of sixty cards. There is no maximum deck size; however, you must be able to shuffle your deck with no assistance.
Two-Headed Giant uses the Unified Deck Construction rules. With the exception of basic land cards, a team's combined decks may not contain more than four of any individual card, counted by its English card title. (For example, if one player is using four Naturalizes in a Multiplayer Constructed event, no other player on that team may have a Naturalize in his or her deck.) Sideboards are not allowed in Constructed Multiplayer tournaments.
If a card is restricted in a particular format, no more than one of that card may be used by the team. No players may use cards that are banned in a particular format.
In addition to the above rules, the following card is banned in Constructed Two-Headed Giant tournaments:
Official information about Constructed formats can be found in the Magic: The Gathering Tournament Rules.