How do we make multiple selections?

Hello, Mesut,   

RadMultiColumnComboBox is not designed to support multiple selection out of the box. RadCheckedDropDownList provides functionality to check items in the drop down area and tokenize them in the text area. Additional information is available here: https://docs.telerik.com/devtools/winforms/controls/dropdown-listcontrol-and-checkeddropdownlist/checkeddropdownlist/getting-started 

However, we have a sample approach demonstrated in our Demo application >> Multi-Column Combo Box >> Settings example for a custom implementation about multiple selection in RadMultiColumnComboBox. It uses a RadMultiColumnComboBoxSelectionExtender component that enables multiple selection. Note that this is just a sample approach and it may not cover all possible cases. Feel free to modify and extend it in a way which suits your requirements best.

I hope this information helps. If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik