display two pie-charts

Hey,

i'd like to compose a clock-like looking plot composed out of two 
circles, each showing the length of a period (to compare them). first, 
to do so, it looked the easiest by using pie(), just puting multiple 
pie-charts over each other. the problem is that once the second pie is 
drawn, it replaces the first one. does anybody know how to add a second, 
smaller pie over an existing one, like a layer?

thanks, lars