using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var list = new List<string>();

        list.Add("11:03:01:003 INFO some event has occurred");

        list.Add("11:03:31:004 DEBUG another event has occurred");

        list.Add("11:04:01:015 INFO third event has occurred");

        var list2 = new List<string>();

        list2.Add("11:03:16:003 INFO fourth event has occurred");

        list2.Add("11:03:32:025 DEBUG fifth event has occurred");

        list2.Add("11:03:54:023 INFO sixth event has occurred");

        var result = list

            .Union(list2)

            .Select(x => 

            { 

                return new { Log = x, Time = TimeSpan.Parse(x.Split(' ')[0]) }; 

            })

            .OrderBy(x => x.Time)

            .ToArray();

        foreach (var log in result){

            Console.WriteLine(log.Time + " => " + log.Log);

        }

    }

}