using System.Collections.Generic;

using System.Linq;

using System;

​

public class Program

{

    public static void Main()

    {       

        var list1 = new List<(string, long)> {

            ("Key1", 2L),

            ("Key2", 1L),

            ("Key3", 1L)

        };

        var list2 = new List<(string, long)> {

            ("Key1", 2L),

            ("Key2", 1L),

            ("Key3", 1L)

        };

        list1.AddRange(list2);

        var resultingDict = list1.GroupBy( d => d.Item1)

            .ToDictionary(g => g.Key, g => g.Sum(d=> d.Item2) )

            ;

        resultingDict.Select(i => $"{i.Key}: {i.Value}").ToList().ForEach(Console.WriteLine);

    }

}