Select one of the small circles then switch to the Fill Tool (F5). From
the drop down list of Fill Types on the top left side of the Infobar, select
Circular. Click on each of the small squares on either side of the fill
path arrow and mix the two colors shown top right. Click on the inside
square and move the center of the fill down to about the lower 1/3.
Apply the same fill type to the other small circle but change the colors.