MAKE THE SEMIFREDDO Line a 9-by-5-inch loaf pan with plastic wrap, leaving a few inches of overhang all around. In the bowl of a stand mixer fitted with the whisk, beat the egg yolks at medium speed until they begin to lighten in color, about 2 minutes. In a small, microwave-safe bowl, stir the honey into 1 1/2 teaspoons of water until dissolved. Microwave at high power until very hot, about 45 seconds. With the mixer at low speed, slowly pour the hot honey into the yolks; increase the speed to high and whip until pale yellow and fluffy, about 8 minutes.
Meanwhile, in a small saucepan, combine the sugar with 1/3 cup of water and bring to a simmer, stirring until the sugar dissolves. Brush down the sides of the pan with a wet pastry brush. Simmer over moderately high heat, undisturbed, until the syrup reaches 240 on a candy thermometer (soft-ball stage), about 10 minutes; tilt the pan if necessary to get an accurate reading.
With the mixer at medium speed, carefully pour the hot sugar syrup down the side of the bowl into the yolks; be careful to avoid the whisk. Beat at high speed until the mixture is very pale and thick and the bowl of the mixer is no longer warm to the touch, about 5 minutes. Transfer the semifreddo base to a large bowl and wipe out the mixing bowl.
In the mixing bowl, whip the heavy cream with the vanilla seeds until medium-soft peaks form. In three batches, gently fold the whipped cream into the semifreddo base until fully combined. Fill one-third of the loaf pan with the semifreddo and smooth the surface. Freeze for 1 hour, or until partially set. Refrigerate the remaining semifreddo.
MAKE THE SORBET Fill a large bowl with ice water. In a small saucepan, whisk the sugar with 1/3 cup of water and the corn syrup and bring to a boil, stirring to dissolve the sugar. Pour into a heatproof bowl and set it over the ice water, whisking until cool. Whisk in the orange and lemon juices and stir until cold. Pour the sorbet mixture into an ice cream maker and process according to the manufacturers’ instructions, until frozen but still spreadable.
Gently spread the sorbet over the frozen semifreddo and smooth the surface. Freeze for 15 minutes. Top with a final layer of semifreddo, filling the loaf pan. (You may have some semifreddo left over; freeze in a separate mold.) Lay a sheet of plastic wrap directly over the surface and freeze until firm, at least 12 hours or overnight.
Carefully unmold the terrine onto a platter and discard the plastic wrap. Using a hot knife, cut the terrine into slices. Garnish with orange slices and serve.