To reproduce:
-add RadMultiColumnComboBox and bind it to some collection. Use the following code:

radMultiColumnComboBox1.MultiColumnComboBoxElement.EditorControl.ShowItemToolTips = true;
radMultiColumnComboBox1.MultiColumnComboBoxElement.EditorControl.ToolTipTextNeeded += EditorControl_ToolTipTextNeeded;

private void EditorControl_ToolTipTextNeeded(object sender, Telerik.WinControls.ToolTipTextNeededEventArgs e)
        {
           e.ToolTipText ="some text";
        }

As a result the tool tip is not shown.

Workaround:use ScreenTipNeeded event instead