Microsoft has added new shape editing tools. They are not shown in the ribbon, so to use them, the first thing you have to do is customize your ribbon.
Adding the combine shapes tools to the ribbon
These tools are among the many hidden features in the 2010 version. Before you can use them, you must first make room for them in the Office 2010 ribbon. This is easy, because in Office 2010, the ribbon can now be fully customized.
1. Open File
2. Click on Options and then Customize Ribbon.
3. In the Customize the Ribbon part of the menu,
4. Select Tool Tabs.
5. Click on Drawing Tools and
6. Select Format.
7. Click on New Group.
8. Then click on Rename.
9. Name it Combine.
10. Confirm by clicking on OK
11. In the Choose commands from menu, select Commands Not in the Ribbon.
12. In the list on the left, for each of the four “Shape” options, Shape Combine, Shape Intersect, Shape Subtract, Shape Union, select the option
13. And click on Add.
14. Confirm by clicking on OK.
Combine different shapes
To try out these features, start by creating a drawing using multiple shapes. This is what we have done here, by superimposing multiple shapes from the Shapes library under the Insert tab:
When you click on a shape, you will now see that the Drawing Tools tool has a new Combine section.
You can see the action of the four options illustrated in the image.
The Shape Intersect tool combines only the common spaces to create a new shape.
The Shape Subtract tool creates “holes” in an existing shape.
The Shape Union tool is probably the most useful command. It combines all the shapes into a single object and makes the edges of each individual element disappear.
The Shape Combine tool keeps only the parts of each object that are not part of the others.
See this page in french: http://presentations-de-vente.com/combiner-les-objets-dans-powerpoint-2010