In a 5- to 6-quart pan over high heat, bring potatoes and 3 quarts water to a boil. Cover and simmer until potatoes are tender when pierced, 30 to 35 minutes.
Drain. When potatoes are cool enough to touch, cut into 1/4- to 1/2-inch chunks.
Meanwhile, in a large bowl, stir together orange juice concentrate, lemon juice, Dijon mustard, and green onions.
Add potato chunks and mix gently.
Add salt to taste.
With scissors, cut lengthwise down the center of the top of each lobster shell. Set each tail, underside down, on a cutting board. Force a heavy knife through the cut in each shell to slice each lobster tail in half. Rinse lobster in shell.
Lay lobster on a lightly oiled grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); close lid on gas grill. Turn once to cook evenly until lobster is opaque but moist-looking in center of thickest part (cut to test), about 7 minutes.
Mound potato salad equally onto plates; top with caviar.
