Double the cone area, keep the power the same = 3dB more.
Double the power, keep the cone area the same = 3dB more.

So running two woofers off the same power will be louder. I would say though, rather than adding extra cone area, you should get a well designed slot ported enclosure built for the single woofer. There is so much to be said for box design and oddly enough it's an area most often over looked with the abundance of pre-fab enclosures available these days.

__________________
John Stamos fan club lifetime member