A "tower" sub will act like a single sub. All 4 drivers will mutually couple and the wavefront from the drivers will propagate as one wave. You won't get any frequency response smoothing from the individual drivers. You'll only get one set of room modes from the entire box. In addition, if you place the other box within a 1/4 wavelength of the first one, that box will also mutually couple with the other box and all 8 drivers will propagate as if they were one big subwoofer. You'll get MASSIVE output, but no frequency response smoothing. IME, dual subs located between the mains and center has never worked. It looks really cool, but it doesn't measure or sound very good.
Putting them in separate boxes allows you to place them around the room for FR smoothing. Putting them in dual-opposed boxes takes advantage of the balanced, opposed forces, which takes cabinet resonation out of the equation. (I would imagine it would take MASSIVE boxes to ensure you get no box resonation whith 4 18" drivers in each box.)
My vote: 4 dual opposed boxes place around the room.
Lombardi said it:
"Perfection is not attainable, but if we chase perfection we can catch excellence."