var numbers1to5 = new[] {1, 2, 3, 4, 5};
var numbers4to8 = new[] {4, 5, 6, 7, 8};

var numbers1to8 = numbers1to5.Concat(numbers4to8);

Console.WriteLine(string.Join(",", numbers1to8));

//1,2,3,4,5,4,5,6,7,8

Note that duplicates are kept in the result. If this is undesirable, use Union instead.