FYI - this is a feature users have been asking for ever since the first generation iPod back in 2001. People have wanted to be able to directly transfer content from iPod to iPod for 9 years now and Apple hasn't given it to the consumers. Why? That is a good way for music piracy to increase. Someone can take their iPod to their high school, hook it up to another one, and copy music files to and from one another without paying. Microsoft introduced a sharing feature with the Zune but you essentially have the file for 5 days. After that, you actually have to buy the song.
Sure, there are some people who would honestly use the feature but they pale in comparison to the amount of people that would use it to pirate music. Lastly, you should have your music stored on your computer as well as your iPod. There is absolutely no excuse these days to have your music just on your iPod. It may sound harsh but I come across at least one person a week who had their music stored solely on their iPod, ran into an issue, needed to restore their iPod, and could not copy the music off of it. So they ended up losing everything. It has been a lesson learned by many. Go buy an external hard drive to store your media on if your computer is running low on space. Either way, you shouldn't rely on your iPod as a permanent means of storing your media (they aren't designed for that).