Toss together cucumbers and 1 ½ teaspoons salt in a large colander placed inside a large bowl. Let stand, uncovered, in refrigerator for 30 minutes.
Meanwhile, evenly divide dill sprigs, garlic, mustard seeds, and dill seeds between 2 clean, heatproof 1-quart glass jars.
Thoroughly pat cucumbers dry using paper towels; evenly divide between jars. Set aside.
Whisk together 2 cups water, vinegar, sugar, and remaining ¼ cup salt in a medium saucepan; cook over medium, whisking occasionally, until salt and sugar dissolve and mixture is boiling, 5 to 7 minutes.
Pour hot vinegar mixture over cucumbers in jars.
Refrigerate, uncovered, until flavor infuses into cucumbers and mixture is cooled, 1 to 2 hours.
Cover jars with lids, and refrigerate for up to 1 month.
