Steam lobsters in a large pot, covered, for 14 minutes. Transfer to a plate; let cool. Remove meat from shells.
Whisk together 2 tablespoons of the butter, melted, with lemon juice; toss with meat. Cut brioche into 1 1/2-inch cubes. Melt remaining 2 tablespoons butter in a medium skillet. Toast bread until golden; toss with sea salt.
and bread between 2 plates.
Serve with lemon wedges
for squeezing, and garnish
with fresh tarragon.