Set up the combo box you want to populate based on your other choice as a query (use wizard).
Use the query
SELECT [column name from table]
FROM [table name]
WHERE [table name].[column name] = [first combo box name]
When this is working, right click on your query combo box, go to properties, events, after update, choose code builder and where the cursor is type 'refresh'. Save and exit the code builder and everything should work perfectly.