2400 cubic ft. main room open to 2000 cubic foot room upstairs.
That’s not so crazy.

I was shopping for subs for a 8000’3 room!
I would recommend two separate subs for sure, rather than one. Also consider subs rated here for large and extreme rooms according to the Bassaholic rating system.
Depending on the floor space you have available, I would always recommend going a little larger if you can. Two 13” or 15” subs, assuming they are good quality, will easily take care of that space. Hsu VTF2s at a minimum, Vtf3 or 15H would be better. Outlaw X13. Svs Pb 3000 or 4000. Svs PC4000! If the size works for you, I would consider 2 15” Rythmik over the 18”.
With the open space that you have, I would definitely suggest looking at ported subs, too.