In a medium bowl, whisk together 2 tablespoons lime juice, 2 tablespoons olive oil, minced garlic, salt, pepper, and red pepper flakes if using. Add shrimp and toss to coat. Let marinate at room temperature for 10 minutes.
While shrimp marinates, prepare cucumbers and onion. Place them in a large salad bowl.
Heat a large skillet over medium-high heat. Add the shrimp along with the marinade. Cook for 2-3 minutes per side until shrimp are pink and opaque. Remove from heat and let cool slightly.
Once cool enough to handle, chop shrimp into bite-sized pieces (about 3-4 pieces per shrimp).
Add chopped shrimp to the bowl with cucumbers and onion. Add cilantro and mint.
In a small bowl, whisk together remaining 1 tablespoon lime juice and 1 tablespoon olive oil. Pour over salad and toss gently to combine.
Serve immediately, or refrigerate for 15 minutes to let flavors meld. Enjoy cold or at room temperature.