well heres a simple little php script that would do it....
Echo it to the screen then just save as .TXT
PHP Code:
<?php
$length = (APPROX FILE LENGTH)
$fa = file("file1.txt");
$fb = file("file2.txt");
for($x=0; $x<$length; $x++)
{
echo $fa[$x] . $fb[$x] ."\n";
}
?>