Rob
We did this at college.However we did not sow the crop using the normal spacings. We simply used a different part in the distribution head which in turn blocked off every other coulter. Did a beautiful job, you would only know it was not drilled with a precision drill because the spacings between plants was not the same.
I am fairly sure the drill was an accord with the normal coulters, but not a combi.
The contractor involved was happy with the arrangement once he had bought a kemper.
However, if you sow the crop at normal row spacings you might get problems at harvest time, since we have found with our jaguar and kemper you sometimes have to slow up a bit when going across the rows otherwise you either get bung ups or you push/cut plants over before they get a chance to feed into the auger. It does depend on how mature the crop is, its yield and if it has got laid or not though. I was sat in the passenger seat once as our head driver went across the rows diagonally at heck of a speed and the crop fed fine.
I'd imagine that so long as the seed rate is the same you should be okay since the volume of crop on the header at any one time is similar.
If in doubt why not just fit the different part like i said? Im not sure if it was home made or a specially produced part from accord.
Perhaps your contractor is keen to get the drilling work?
Mayo