Place the lentils in a medium saucepan and enough water to cover by about 2 inches. Bring to a boil over medium high heat and simmer until tender, about 10 to 15 minutes. Drain well.
Transfer the lentils to a bowl while they are still hot, and then stir in 2 tablespoons of the vinegar. Season to taste with salt and pepper. Cool lentils, stirring occasionally.
Place the 2 cups of water and ½ teaspoon salt in a small saucepan. Bring to a boil over high heat and add the couscous. Cover with a tightly fitting lid and remove the saucepan from the heat. Let couscous stand, covered, for 5 minutes.
Fluff couscous with a fork and transfer to a large bowl. Stir in 2 tablespoons of the olive oil. Cool couscous, stirring occasionally.
In the meantime, place the grated garlic, remaining vinegar, and remaining olive oil in a small bowl. Whisk the dressing until well combined and season to taste with salt and pepper.
When the lentils and the couscous are completely cool, add the lentils to the couscous and then pour over the dressing. Gently stir everything together and cover with plastic wrap. Chill in the refrigerator for at least 3 hours and up to 24.
When ready to serve the salad, stir in the cherry tomatoes, spinach, mint, and feta. Season to taste with salt and pepper. Serve chilled or at room temperature.