I BELIEVE (not 100% positive though) when you install a theme over another theme it works ok. I think anything that wasnt changed by the second theme remains from the first (ie: first theme had a themed power control widget and the second theme didnt, well now your on the second theme but with the themed power control widget from the first one... if that makes sense to you)
not all themes have EVERYTHING themed, so you probably will see some overlap.