Thirdly I would check to see if other vans that have the double seat even have the passenger airbag, it may be if the airbag is not suitable for two people then it is not installed when the van is build.
Already the passengers' airbags are quite rare in Ducatos/Jumpers/Relays/Boxers, as they are mainly commercial vans. But it was an option on both two- and three- seaters for the x250.
There's no place to put an airbag in front of the middle seat.
Are you sure the existing passenger airbag does not also deploy over the centre area?
In general the passenger's airbag is bigger than the driver's one. And inflates quicker
On the attached photos:
three seater with the passenger's airbag,
three seater without the passenger's airbag,
two seater with the passenger's airbag,
again same van as on the first photo: three seater with the passenger's airbag,
two seater without the passenger's airbag,
and finally another two seater with the passenger's airbag.
First and fourth photos are from the same van. It is very well equiped - leather steering wheel and gear knob, webasto parking heater, cruise control, radio controls on the steering wheel, just to mention the few visible factory fitted elements.