Cut each scallop into 8 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 gently so each piece is given a good lime juice bath. Cover the bowl with plastic wrap and place the bowl in the refrigerator to marinate for 4 to 10 hours. The shellfish is done when 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 3 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 the mixture with the salt and mix well. Cover the bowl with plastic wrap and 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 gently toss together. Taste the seafood and add more salt or fresh lime juice, if desired.
For a knockout presentation, spoon the seafood ceviche into the coconut shell halves and top with a sprinkle of unsweetened coconut, if desired. Pile warm tortilla chips around the coconut shells, if desired.