I haven't used this GE in several years, but just took a quick look at it. What you need to do is complete the edit in Expert Mode. You will see that the file is now made up with several layers. The extended arms/legs are set on the layer with the black mask (second from the top). You can use this mask to paint over the second leg (or arm) with a white brush. Here is a crude example. If you need more help in understanding the mask, let us know.
Sorry for the late reply. In the PSE "With Instructions" after having chosen the frame then it seems that only the Quick Selection Tool is selected. There is a Tool Option area but can not change the type of selection. I experience sometimes that I can not make an accurate selection. So as you wrote I went to Expert mode and I used the white brush to select all what should be visible outside the window. And what is selected too much outside the window I switched to the black brush.
But is there any possibilty to select another type of selection tool other than the Quick selection in the "With Instructions" mode?
(In fact, it doesn't really matter in which mode you complete the selection in)