Two-Headed Giant 2PAC follows the Pacific Old School rules with the following exceptions:
With the exception of basic lands, a player’s deck may not contain more than 2 of any individual unrestricted card counted by its English card title equivalent.
Black Lotus and Chaos Orb are unrestricted (so you can play 2 of each). If you choose to play a 2nd copy of either of card, the 2nd copy can be a "tasteful" proxy (be reasonable).
There are no sideboards.
Matches consist of a single game and are played in teams of 2 vs 2. Teams are generated randomly each round based on individual win/loss record from prior rounds. Each teammate brings their own legal 60-card deck.
Official Wizards 2HG rules will be used, but here is a summary of some key differences: