using System;
using System.Collections.Generic;
using System.Linq;
public class Program
{
public static void Main()
List<string> list1 = new List<string>(){
"4041",
"4040"
};
List<string> list2 = new List<string>(){
"4039",
"4044",
"4075",
"4010",
"4100",
"4070",
"4072"
list1
.SelectMany((x,idx) => new[] { x, list2[idx] })
.Concat(list2.Skip(list1.Count)).Dump();
}