Put the crab legs in the bottom of a large stockpot, then cover with 4 quarts of water and stir in the seafood seasoning (see Cook's Note). Halve the lemons and squeeze the juice into the pot, then add the rinds. Cover and bring to a boil, then cook over medium-high heat until the crab legs are warmed through, about 5 minutes. Remove the crab legs to a large serving bowl or platter.
Meanwhile, melt the butter in a small saucepan over medium-low heat. Cook until the butter foams and simmers, about 1 minute. Remove from the heat and let stand 5 minutes. Using a spoon, remove any milk solids from the surface of the butter, then slowly pour the melted butter into a small serving bowl, leaving any watery, white whey behind in the saucepan. Serve alongside the crab legs.
Depending on the size of your stockpot, you may need to add more water to cover the crab legs. If that is the case, add 1/4 cup more seafood seasoning and 1 more lemon for every additional 2 quarts of water.
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