multiple contour_plot/implicit_plot on the same graph

Hi,
Is it possible to put in one graph more than one contour_plots (with the
same domain), like
  contour_plot(sqrt(x^2+y^2),[x,-10,10], [y,-10,10],[legend,false]);
  contour_plot(x^2+y^2, [x,-10,10], [y,-10,10],[legend,false]);
?  Ditto for implicit plots.

Thanks!