Here is what I have done in the past...
Combo 1 is the first one you would select from.
On the OnClick event for Combo 1 have it do Combo2.Requery
For Combo 2, have the record source as a SQL Statement, but with a WHERE clause that references Combo 1.
Below is an example of 1 I did a while back.
SELECT tblSubData.SubData, tblSubData.SubDataID, tblSubData.SubDataParent FROM tblSubData INNER JOIN tblData ON tblSubData.SubDataParent = tblData.DataID WHERE (((tblSubData.SubDataParent)=[cboCombo1].[Value])) ORDER BY tblSubData.SubData;
Hope that helps - If not yell!