Random rnd = new Random();           

int[] MyArray1 = { 1, 2, 3, 4, 5, 6, 7, 8 };
int[] MyArray2 = { 10, 20, 30, 40, 50, 60, 70, 80 };           

for (int i = MyArray1.Length - 1;  i >= 1; i--)
{
    int j = rnd.Next(i + 1);

   (int, int) tmp = (MyArray1[j], MyArray2[j]);
   (MyArray1[j], MyArray2[j]) = (MyArray1[i], MyArray2[i]);
   (MyArray1[i], MyArray2[i]) = tmp;
}            

(int x, int y)[] MyArray3 = new (int, int)[MyArray1.Length];

for (int i = 0; i < MyArray1.Length; i++)
{
    MyArray3[i].x = MyArray1[i];
    MyArray3[i].y = MyArray2[i];
}

Console.Read();

class Program
    {
        static void Main(string[] args)
        {
            Random rnd = new Random();

            int[,] MyArray1 = { { 10, 20, 30 }, { 11, 21, 31 }, { 12, 22, 32 }, { 13, 23, 33 }, { 14, 24, 34 }, { 15, 25, 35 }, { 16, 26, 36 }, { 17, 27, 37 } };
            int[] MyArray2 = { 0, 1, 2, 3, 4, 5, 6, 7 };

            int[] ArrayOfIndices = Enumerable.Range(0, 8).ToArray();//.OrderBy(x => rnd.Next()).ToArray();

            //Вариант 1 - Работает.
            (int[] x, int y)[] MyArray3_1 = new (int[], int)[MyArray1.GetUpperBound(0) + 1];

            for (int i = 0; i < MyArray1.GetUpperBound(0) + 1; i++)
            {
                MyArray3_1.x = new int[MyArray1.GetUpperBound(1) + 1];

                for (int j = 0; j < MyArray1.GetUpperBound(1) + 1; j++)
                {
                    MyArray3_1[i].x[j] = MyArray1[i, j];
                }

                MyArray3_1[i].y = MyArray2[i];
            }

            //Вариант 2 - Работает.
            (int[] x, int y)[] MyArray3_2 = new (int[], int)[MyArray1.GetUpperBound(0) + 1];

            for (int i = 0; i < MyArray1.GetUpperBound(0) + 1; i++)
            {                
                MyArray3_2[i].x = Get_row(MyArray1, i);         

                MyArray3_2[i].y = MyArray2[i];
            }

            //Вариант 3 - Не работает.
            (int x[], int y)[] MyArray3_3 = ArrayOfIndices.Select((element, index) =>
                (x:  Get_row(MyArray1, index), y: MyArray2[index])).ToArray();
            

            Console.Read();
        }

        static int[] Get_row(int[,] A, int i)
        {
            int num_columns = A .GetUpperBound(1) + 1;
            int[] row = new int[num_columns];

            for (int j = 0; j < num_columns; j++)
            {
                row[j] = A[i, j];
            }

            return row;
        }
    }