Do you mean the table commonly called "bill of materials?"
If so just insert a column and name it then select the custom property from the drop down. The dual column, two line setup you seek probably can't be done because of the structure of the SW BOM, so just insert two columns. You might be able to do this with an excel BOM.
Maybe you can fake it with a second header line? Click the ^^ to turn it into vv over in the leftmost column of the BOM while in edit mode. Place your two quantity columns at the far right, then right justify your header top line.
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Hardie "Crashj" Johnson
SW 2011 SP 2.0
HP Pavillion Elite HPE
W7 Pro, Nvidia Quaddro FX580