This will give you problems, as I'm sure you know, because there can be only one Control Source for a combo box, i.e. only one destination field.
Off the top of my head, the way to accomplish this would be to use code to take the two values from the combo box and write them into a new record. This would run after the user had selected a row in the combo box, or perhaps after they have pressed an "OK" button the confirm the operation.
Let's look at this in more detail. If your combo box is called CmbRes, and you have Res_ID and Res_Line_ID as the first two columns, the code section that writes the record into TblOrders would look something like:
myset!Res_ID = CmbRes.Column(0)
myset!Res_Line_ID = CmbRes.Column(1)
...where "myset" is the name of the recordset you've opened on TblOrders.
The important thing to note here is that you can reference any column of a combo box (or a list box, for that matter) by its ordinal number, where the first column reading from left to right is always column zero. Hope this helps!!
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