When you use a template for the bar visuals there is virtually no way for the chart to know what color it should pick for the legend item. I think that the easiest way to go here is not to set legend settings for the bar series. This will result in no legend item displayed for the bar series. Then you can manually place a RadLegend with one legend item, above or below the other RadLegend in a stack panel.
Another approach would be not to use a template for the bar visuals but use the DefaultVisualStyle property of the BarSeries instead. If this is possible, this is the solution I suggest.
the Telerik team