Not only are you allowed to share cards, you are as a team considered to have a single card pool that you build your decks from. SectionsSection 9.6 and 9.7 of the Tournament rules saysays
Two-Headed Giant Limited Rules
All the rules for Limited Tournaments (Section 7) apply, except as described below.
The DCI recommends that each team receive eight boosters per team for Two-Headed Giant Sealed Deck tournaments and six boosters per team for Two-Headed Giant Booster Draft tournaments. For the recommended product mix for the current block, refer to Appendix D.
Cards not used in a team’s starting decks are considered a shared sideboard by the two players that both players can access.
Two-Headed Giant Booster Draft Tournaments
Teams (not players) assemble into random drafting circles (called pods) of roughly equal size at the direction of the Head Judge. Teammates sit next to each other. Tournament officials then distribute identical booster packs to each team in the pod.
After opening and counting the cards in their first pack, the team chooses two cards from the booster pack then passes the remaining cards face down to the team on its left. Selected cards may be placed into one or two piles. The cards chosen are not assigned to a particular player; they become part of a pool out of which both players will build their decks. The open packs are passed around the drafting pod—with each team taking two cards from each before passing—until all cards are drafted.