Note: At the bottom of the right side menu you can check a box that will import the second PowerPoint's slides in the same style you created them. If you leave it unchecked the slides you import will be changed to the first PowerPoint's theme and style.
At this point you should be able to see both PowerPoint slide shows in side menus, one on each side of the main window.
To add the slides from the right PowerPoint to the left PowerPoint, place your cursor and click on the space between the slides where you want the new slide to go. You will see a dark line appear where you clicked. Next, click on the slide in the right menu that you want to add. Clicking on it will add it to your first slide show.
To rearrange the slides in the first slide show just click and drag the slides up or down to change their position.