You are right that iDVD supports only one voice track. But iDVD is the last stage of an assembly line.
For slideshows, you can either do them in iMovie, or in iDVD. If you use iMovie, then again, just add the voiceover directly, then import as a video track into iDVD.
If you choose to do the slideshow in iDVD, you'd set it up, including the background music. Then play the slideshow, and simultaneously record the voiceover (i.e. start recording in a simple recording app a bit before you start playing). Then use Quicktime, Garageband, or iMovie to mix the audio tracks. Then reapply the new mixed audio track to the slideshow.
You definitely want to be using iDVD for this, though; it makes DVD creation a breeze.