The power rating will double, both in series or in parallel (only the impedance changes), two 40W drivers give 80W handling.
I presumed you were talking about much larger drivers?, these aren't really sub-woofer material, more like mid-range drivers?, or bass/mid in very small systems.
Notice that it's a screened speaker, so it's intended for TV use, and suggested as a centre speaker - where there is no bass!.
I'm also rather confused about the tweeters?, you've mentioned a sub-woofer, and tweeters, but nothing else?.