How to Make It
Process tomatillos in a blender until smooth, about 1 minute. Pour through a fine wire-mesh strainer into a bowl. Discard solids.
Return strained tomatillo juice to blender; add habeneros and oregano. Process until smooth. Transfer to a small bowl, and set aside.
Cook garlic and 2 tablespoons olive oil in a small saucepan over medium-high, swirling occasionally, until garlic is golden, about 3 minutes and 30 seconds. Remove from heat, and let cool 15 minutes. Remove and discard garlic clove. Add garlic oil and remaining 2 tablespoons olive oil to tomatillo mixture; stir to combine. Stir in salt to taste. Chill until ready to use.
Cook shrimp in a pot of boiling water until just done and pink, about 3 minutes; rinse immediately under cold water. Stir together shrimp, scallops, Key lime juice, and salt to taste in a large bowl. Cover and chill 30 minutes or up to 2 hours.
To serve, divide tomatillo-habanero sauce evenly among 4 shallow bowls; arrange shrimp mixture on top. Garnish with radishes, cucumber, and cilantro.