You can achieve the desired functionality, using the OnClientItemChecked client-side event of the RadComboBox and the Client Api of the control. Please refer to the following article, providing more information on the Client Api of the control:
However, please keep in mind that such altering is not a supported functionality.
In the attachment you can find an example on how this could be achieved. You can use the first and last index of the checked items collection, in order to get the text of those items and set the text of the combobox. In addition, I would suggest you to store the text of the combobox, after the alternation, in a hidden field, in order to re-set it in the OnClientLoad of the control and thus make sure that the text is persisted after a postback on the page.
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