Make the citrus butter: stir together the butter, lemon and lime zests, lemon and lime juices, and salt in a small bowl until mixture is well blended. Divide the mixture equally onto 4 sheets of plastic wrap; shape each into a flattened square. Wrap the squares tightly in the wrap, pack them in a zippered plastic bag, and store them in the freezer until you are ready to use them. To use them, cut each square into quarters. Each quarter will be about 1 tablespoon.
With a knife, score the skin of the magret in a criss-cross pattern, making the squares as small as possible without cutting into the meat. Season with salt and pepper on both sides.
Heat a pan over high flame. When hot, place the duck breasts skin side down and reduce heat to medium. Cook for 8 minutes, while continuously draining off the rendered fat. Flip over and cook for 4 minutes on the meat side.
Remove the duck breasts to a cutting board and tent with foil while resting for at least 5 minutes.
Slice magret in 1/4 inch slices. Lay slices on warm plates in a fan pattern then top each portion with a square of citrus butter. Serve immediately.