in windows XP i have the taskbar at the bottom screen edge, double height, with the XP QuickLaunch toolbar occupying the top row and the task switcher tabs occupying the bottom row. is there a way to achieve the same effect in KDE4.x? either by stacking two panels or by some other widget that can do either of the two jobs, and can be stacked?

At this time, it is not possible to have a two row panel. Someone would need to write the appropriate Plasma Panel Containment to do what you desire.