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. Section 9.6 of the Tournament rules says
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.