using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var array1 = new string[,]{ {"a","1"}, {"b", "2"}};

        var array2 = new string[,]{ {"a","3"}, {"c", "3"}};

​

        foreach(var t in Program.ToTuples(array1).Concat(Program.ToTuples(array2))

          .GroupBy(row => row.Item1)

          .Select(t => (t.Key, t.Sum(row => row.Item2))) ) {

            Console.WriteLine(t.Item1 + " " + t.Item2);

        }

    }

    static IEnumerable<(string, int)> ToTuples(string[,] array) {

        for (int i=0; i < array.GetLength(0); i++) {

            yield return (array[i,0], int.Parse(array[i,1]));

        }

    }

​

}