using System;

public class Program

{

    public static void Main()

    {

        string str = "this <strong>is</strong>[separator]<strong>a test of the space</strong>";

        string[] els = str.Replace("<?/strong>","").Split(new string[] {"[separator]" }, StringSplitOptions.None);

​

        Console.WriteLine(els[0] + " " + els[1]);

    }

}