Hi Fady,
This can be achieved by using the CurrentCellCloseDropDown event. In the event, the datasource / cellvalue of the second column can be changed depending on the value selected in the first combobox column.
Refer to this forum thread for more details :
Forum_42073
KB article :
How do I change the DataSource of one ComboBox cell depending upon the value selected in another ComboBox cell in a GridDataBoundGrid?
Best regards,
Madhan