Perhaps you could try the free version of Wavepad? I found it much easier to use than Audacity, myself. Just select what you want, copy it, and paste it onto the other file. 'Tis like a word processor. I think there is also a join function, if you prefer to do it the automatic way...
http://www.nch.com.au/wavepad/
I am reading the Audible FAQs, though, and it seems like their DRM might prevent something like this from happening at all. Still, worth a shot.
http://audible.custhelp.com/