How does Dread work in a Two-Headed Giant game?
Comprehensive rules, 810.9:


  [In a Two-Headed Giant game, d]amage, loss of life, and gaining life happen to each player individually. The result is applied to the team’s shared life total.


If Player A has a Dread on the battlefield, "you" refers to its controller, Player A109.5.  Dread's ability will only trigger if player A is dealt damage.  Attacking A's partner will not trigger the ability.