I don't believe that this is possible within the ASP.NET Ajax RadChart, but you can always submit this as a feature request in PITS
What you could potentially do is look into the RadChart for Silverlight that Telerik offers. This can be integrated quite easily into an ASP.NET Ajax application (as seen here
) and if you take a look at this demo
you can see that when you hover over particular bar another Chart appears as a tooltip. Although this does not fully represent what you want to do, perhaps having a bubble chart with the Pie charts as tooltips could be a good alternative.