Hello,
A merging class is available in the current testing branch and will be included in the next stable release of phppptx (there's no release date). This new feature is fully tested and working.
If you purchase a license (Advanced or Premium), please send a message to contact[at]phpdocx.com and we'll send you the new class.
A sample merging PPTX files:
require_once 'classes/MergePptx.php';
$pptx = new MergePptx();
$pptx->merge(array('file1.pptx', 'file2.pptx', 'file3.pptx'), 'output_merged.pptx');
There's no limit on the number of PPTX files to be merged.
Regards.