It appears that when you add a label to any of the available values, even if it is a Boolean, it defaults to a dropdown list. If you create a boolean without labels. this will keep the radio buttons, then you could make the "Prompt" something like "Select True for Top 2, False for Top 10". Not the prettiest, but will work.
As an alternate solution, you could create a report without a parameter, and make a drilldown report. The base report would be for your Top 2, and the drilldown the Top 10. The drilldown toggle crosshair would act like your radiobutton, just some food for thought.