1. Select the set of UVs that will be your clone source in the UVs section of the Object Data panel.
2. With that selected, go into Edit mode on your mesh and bring up a UV eDitor window.
3. Make sure that the Clone Source UVs are showing in the Editor, and open your source image there.
4. Leave edit mode.
5. Select your target set of UVs, repeating the process in the UV Editor so your target (blank) image is bound to the target UVs
6. Go into Texture Paint mode on the model.
7. In the brush panel on the tool shelf (T-key), select the Clone tool
8. Enable the Project Paint panel of the tool shelf.
9. Enable Clone on the same panel.
10. To the right of the word “Clone” will be the name of one of your UV sets. Clicking on that name allows you to select any of your available UV sets.

Really, the whole thing’s so obvious I’m shocked that everyone can’t do it.

taken from

http://blenderartists.org/forum/showthread.php?t=174016