for error 500 : it could be a pb of memory-limit. try to ecrease it in php.ini

for merge multiple pdf file : no sorry...

but try like this :

$content1 = '.....';
$content2 = '....';
...

$html2pdf = new HTML2PDF(...);
$html2pdf->writeHTML($content1);
$html2pdf->writeHTML($content2);
....
$html2pdf->output();