In general, you would need to use VBA to use the Table object and cycle through all the fields, assigning that list to the second combo box. The challenge is getting the list of fields into a recordset that is usable by the combo box. The only way I know how to do that is by using an ADODB recordset.
But, other than this advice, I am not going to build this for you when you have not put forth any effort to work through this yourself or show any of your work thus far.