Make sure you have an engineer inspect your foundations first!
I don't think bipoles are too much different then monopoles for subs. The wavelengths involved are so long the extra path length shouldn't make that much of a difference. As far as advantages - it can be a more compact way to set up your subs. Also the vibrations from each driver should cancel each other out.
My only possibly negative thought would be that 3" may not be enough clearance for the rear four 18" drivers.