That control is designed to work with a single product only. The code expects there to be only one of everything. The reason for it being a RadListview is to get databinding in the widget templates so anyone could drag / drop any property of the product they want.
You could override InitializeControls and tie the button click to a different event handler but you'd have to figure out how to know which product should be added to the cart. Plus it only accepts a single GUID for a product Id.
I suppose it would be possible but it would probably be easier to make your own control.
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