I did it now :) i read about word typelib, but the including didnt work so I read out all the constants and defined them myself. I dont wanna use LibreOffice if i dont have to because creating the docs with word, then analyse them with libre and then convert them with word are too many components that can go wrong i think.
So here is my function. I use it on every created document before merging:
public function printDuplex($file){
            $word=new COM("Word.Application") or die("Cannot start MS Word");
            $word->visible = 0 ;
            //6 = wdStory VB (end of document)
            define('endOfPage', 6);
            //(2 = wdStatisticPages VB)
            define('pageCount', 2);
            //open File
            $word->Documents->Open($file);
            // get pagecount
            $count = $word->ActiveDocument->ComputeStatistics(pageCount);
            //If odd pagecount
            if($count%2 > 0){
                //6 = wdStory VB (end of document)
                //go to end of document
                $word->Selection->EndKey(endOfPage);
                //add page
                $word->Selection->InsertBreak();
            }
            //save document
            $word->Documents[1]->SaveAs($file);
            $word->Quit( );
            $word = null;
        }
Thank you for your help you sat me on the right track :)
regards