You dont have to parallel the drivers you can have isobaric with the drivers in series. If you look the simulation the 4th order isobaric with the same Pioneer driver gives you approximately 8 db more at 50hz in a rougly 20 liter box while one closed subwoofer would need 47 liters. What is the more efficient design? I am not assuming anything i have built such a subwoofer.You showed me an example. How do you know that its an efficient design? If you have the data please share, otherwise you are just assuming. I am instead showing you the math why your assumption is not correct.
Meanwhile, when you parallel the drivers you reduce the resistance hence increase the current drown from the amp. No difference to my suggestion of using an EQ, which increases the voltage output of the amp. Same battery load.
It is always better to use simple math than assume blind.