import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

import org.apache.poi.xslf.usermodel.XMLSlideShow;
import org.apache.poi.xslf.usermodel.XSLFSlide;

public class MergingMultiplePresentations {
   
      public static void main(String args[]) throws IOException {
      
      //creating empty presentation
      XMLSlideShow ppt = new XMLSlideShow();
      
      //taking the two presentations that are to be merged 
      String file1 = "presentation1.pptx";
      String file2 = "presentation2.pptx";
      String[] inputs = {file1, file2};
      
      for(String arg : inputs){
      
         FileInputStream inputstream = new FileInputStream(arg);
         XMLSlideShow src = new XMLSlideShow(inputstream);
         
         for(XSLFSlide srcSlide : src.getSlides()) {
         
            //merging the contents
            ppt.createSlide().importContent(srcSlide);
         }
      }
     
      String file3 = "combinedpresentation.pptx";
      
      //creating the file object
      FileOutputStream out = new FileOutputStream(file3);
      
      // saving the changes to a file
      ppt.write(out);
      System.out.println("Merging done successfully");
      out.close();
   }
}

$javac MergingMultiplePresentations.java
$java MergingMultiplePresentations

Merging done successfully