foreach line in file1...
{
  loop through lines in file2...
  {
     ...many comparisons... decide which one is next
  }
}

##</code><code>##

foreach line in file1...
{
    # insert file2 line(s) that should go before the
    # current line from file1
    while {the current line in file2 is "next" in total order}
    {   
        output that line from file2; 
        move to next line in file2
    }
    output the file1 line
}
output rest of file2 lines (if any)