The default control does not support this functionality. You will need a custom control that will allow you bulk download. You should create a custom control that uses RadGrid with AllowMultipleSelection property set to true. You will have a property of type Guid where you will save the IDs of all selected Content items ( documents) Outside of the Grid you will have a LinkButton control. When you click on this button you should get the IDs of each item from Guid array, call LibraryManager and force downloading the items by appending download=true to each link. Generally you need to implement custom control with group operation.
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