Suppose you have the option group with the two radio buttons. Suppose the option group is called grpCodeGroup and the two options buttons are called optTable1 and optTable2. When you click optTable1, then grpCodeGroup will equal 1. When you click optTable2, grpCodeGroup will equal 2. On your option group's AfterUpdate event, you have something like this:
If Me.grpCodeGroup = 1 Then
Me.cboMyCombobox.Rowsource = "SELECT [Field] FROM Table1 WHERE..."
Me.cboMyCombobox.Rowsource = "SELECT [Field] FROM Table2 WHERE..."
For each row source, change the [Field] and WHERE clause to specify what you want.
Michigan Dept. of Environmental Quality, Air Quality Division