interesting idea, thought about it myself.
i get the impression the throat of the horn moves depending on the  frequency the sub is producing, as far as i can tell this may shift as far as the last horn segment, i only say this as ive been finding the wo a bitch to time align correctly with other horn loaded pa speakers.
technically a delay of 1.57m would bring the rest of the pa inline but it doesnt seem to be the case (if anyone has input to this im interested to hear it)

aanyways due to the possibility that the wo gets such increased gain at low frequencies from both drivers powering one throat at occuring at some point after the 2nd bend /v divider. you may lose out. however as you rightly said it would have a bigger horn mouth so maybe that would compensate, or maybe the throat doesnt move as much as i think it does , if thats the case then your design should work well.

if moving and stacking the box isnt really a consideration id suggest building it as you suggest , one side of a wo mirrored, id say the dividing wall between the 2 sides could be left out aswell,did a picture in paint to illustrate this but it wont upload.
also consider extending the horn flare 6-10" to fully couple both horn mouths into one , could well end up being a good box

due to not being able to accurately model this box i reccomend building a prototype out of mdf as its dead cheap and can always be painted e.t.c if you decide to keep it.

might have to try and work out a way to buld a wo like this but with decent stacking/packing dimensions, could be good.

incidentally had 2 12" loaded (pa drivers) wo's out at the weekend, with 1200 w@ 4r per box, room was about 6x6M and they nailed it, these boxes are cool

One other thing to account for is the fact that the drivers energy and the mouth of the box are about 180 degrees out of phase, (The box makes a u-turn inside), which at 34Hz half wavelength is about 5 meters. Add to that any group delay encountered from the 4th order loading scheme and it's not surprising that a large delay would be necessary.

I find that starting with the drivers 180 degrees out of phase, electrically, with the rest of the system helps a lot. It sounds stupid, redundant, and obvious, but phasing is a relative thing.