Need to Combine Powerpoints?

Now you can! Just follow the steps below:

Open the main presentation and then click between the two slides you want to import the slides into. It’s worth noting  that this method also lets you pick and choose which slides you want to insert from an external presentation, whereas the Insert Object method will insert the entire presentation.
Now click on the Insert menu and then click on the words New Slide and you’ll see a menu appear. At the very bottom of that menu is an option called Reuse Slides

 

When you click on that, a tab will appear on the right hand side. Click on the Browse button and then select Browse file.. Check the Keep source formatting box if you want keep the theme and text formatting of the external presentation.
If you uncheck that box, then when you go to insert the slides into the main presentation, the original formatting will not be kept. Instead the slides will use the theme and formatting of the main presentation.



Select the PowerPoint Presentation you want to insert and then click OK. Immediately, you should see a small thumbnail and the title of each slide displayed.


As you can see above, the second presentation has a different theme than my first presentation. There are a number of things you can do at this point. Firstly, if you just want to insert one slide or a couple of slides from your external presentation, simply click on the slide and it will be inserted!
The formatting will depend on whether you checked the Keep source formatting box like I explained above. As shown below, I inserted just one slide from the external presentation and kept the source formatting.


It will be inserted wherever you had clicked earlier with the red line. If you want to insert all the slides, just select the Insert All Slides option that comes up when you right-click on any of the slides.
Lastly, you can apply the theme used in the external presentation to your main presentation by right-clicking and choosing Apply Theme to All Slides. This will apply the external presentation theme to the main presentation.
As mentioned earlier, you lose all animations, effects, transitions, etc when using this method.


Thanks to: http://www.online-tech-tips.com/free-software-downloads/merge-powerpoint-ppt-files/     for this tip