I've got one iPod touch, but two iTunes accounts on two separate computers
The issue isn't really iTunes accounts, but rather the syncing — you can only sync an iPod or iOS device with one library at a time.

The easiest solution now that iTunes songs are DRM-free is to copy your songs to her library, and then sync her device to her own library. To locate the actual files, select what you want in iTunes, right click and choose Reveal in Finder.