If the scallops are particularly thick, you may want to slice each scallop horizontally into two thinner disks. Cut each scallop into eight wedges. Cut the peeled shrimp into chunks that approximately match the size of the scallop wedges (unless you have rock shrimp, which doesn’t need to be cut). In a medium bowl, pour enough lime juice over the scallop and shrimp pieces to cover. Toss so each piece is given a good lime juice bath. Cover and place the bowl in the refrigerator to marinate for 4 to 10 hours. The shellfish is done with it’s completely white and opaque and doesn’t look or taste raw.
In a separate bowl, mix the remaining lime juice with the lemon juice, coconut milk, and olive oil. You can chop the jalapeno, cilantro, and garlic by hand but if you have a food processor it’s much easier to pulse the three together. Add the pepper mixture to the coconut mixture, and stir in the red onion. (You may add the mango chunks here if you like, but I prefer to add them right before serving.) Season with salt and mix well. Refrigerate for several hours or overnight.
When every chunk of shrimp and scallop is white with no hint of translucence, remove them from the juice. Discard the lime juice. Combine the seafood and the coconut milk mixture, tossing well. Add the mango chunks and toss gently. Taste and add more salt or fresh lime juice, if desired.
For a knockout presentation, spoon the ceviche into the coconut shell halves and top with a sprinkle of unsweetened coconut if desired. Pile warm tortilla chips around the coconut shells.