Change the rowsource of the second combobox to something like
SELECT .........................................
FROM ........................
WHERE (((NameOfTable.NameOfField)=[Forms]![FormName]![cbo1]));
Then on GotFocus event of the second combobox put the code
cbo2.Requery
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/information-technology/12459-multiple-combo-boxes.html