Could you specify what exactly you mean by "multiple selection" when referring to the items of the RadToolBar. In general multiple selection functionality is used when working with controls like RadListBox and RadCombobox, which represents a list of items. On other hand the items of the RadToolBar are usually buttons. Can you specify how you need to use them, so we could recommend a control that is suitable for your scenario?
Any additional information provided would help us point you to the right direction.
the Telerik team