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.
Dess | Tech Support Engineer, Sr.
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