Select the last slide in the first presentation. On the Home menu at the top, click the drop-down menu under New Slide and select Reuse Slides. A pane will open on the right side of the screen which will let you open the file containing the second presentation. Then just click the slides you want to add. Check the box at the bottom of the pane to "Keep Source Formatting" if you want the added files to remain in their original design. Otherwise they will take on the design of the presentation you are adding them to.
