How would stacking two of them in the same place be better than having a sub with with dual drivers?
More internal cabinet space.....
A unit with dual drivers, ususally is a compromised design for size....
Like the Plus/2, is only a small amount of increased interal space over a single Plus unit, but contains 2 drivers, effectively reducing the tuning FR.
The Natural tune of a single Plus is 20hz, where the natural tune of the Plus/2 is 25hz. Plugging 2 ports yields a 16hz tune on the Plus/2 where plugging 2 ports on the Plus yields a tune of 12hz... its more efficient with a larger enclosure.
Like the JL Audio F113, dual F113 units will gain a small amount of SPL over the Gotham.... Personally I would rather have dual non-collocated units to smooth room response over stacked anyday... Any and all of these units will likely produce more SPL then most normal people would need, and would be better utilized in smoothing out the room nodes that is a more common problem then not...