using System;

using System.Text;

​

string combindedString = @"Hello world

6/16/2014

Hi everyone

5/16/2014

This is test

5/16/2014";

string[] lines = combindedString.Split(new string[] { "\n", "\r\n" },

                     StringSplitOptions.RemoveEmptyEntries);

​

var result = new StringBuilder();

for (int i = 0; i < lines.Length; i++)

{

    if (i % 2 == 0 && i != 0)   // i != 0 to not get blank line at the beginning

        result.AppendLine();

    result.AppendLine(lines[i].Trim());

}

combindedString = result.ToString();

combindedString.Dump();