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

var numbers1to8 = numbers1to5.Union(numbers4to8);

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

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

Note that duplicates are removed from the result. If this is undesirable, use Concat instead.