Well, this is a little trickier, but doable. It still sounds like you would do well to have two tables, a look-up table with just your departments and then a department details table so that you can get that one-to-many working (one department, many details). However, you can still do all of this in one table:
-Write a query to capture these two fields. Change its "Unique Values" property to "Yes". That way, you'll have each Department listed only once, no matter how many times it's in the underlying table.
-You still want to have two text boxes bound to the two fields in the underlying table. If you prefer, you can make these boxes invisible, just so long as they're on the form and are being populated with what's being selected in the combo. After this, the After_Update code should get what you're looking for.