If necessary, trim all but 1/8 inch fat from meaty portion of racks. Place each rack, meaty side up, on cutting board. Using sharp paring knife, cut line across rack where meaty portion begins, about 3 inches down from rib ends. Cut off meat and fat between bared ribs to expose bones. Holding each rack firmly, scrape exposed rib bones clean.
In small dry skillet, toast cumin seeds over medium heat, shaking pan often, until fragrant and slightly darkened, about 5 minutes. Let cool. Transfer to spice grinder; grind coarsely. Transfer to bowl and mix in oil, garlic, salt and pepper. (Or, using mortar and pestle, pound seeds coarsely. Add garlic and pound to make thick smooth paste. Gradually blend in oil, then salt and pepper.)
Evenly spread paste over meaty side of racks, pushing some between bones. Spread remaining paste on bone side. Place on greased rack in small roasting pan. (Make-ahead: Cover and refrigerate for up to 1 day. Let come to room temperature before roasting.)
Roast in 400°F oven until lamb is crusty and browned and meat thermometer registers 140°F for rare, about 30 minutes, or to desired doneness.
Transfer to carving board; tent with foil and let stand for 5 minutes. Cut between bones to serve.