It was going to be that the Engineer certifying the ARC would have been an independant Engineer within the company or that of a separate company such as happens now with Airlines.
But I believe it was realised that this would be untenable for one man Companies or those stuck out in the back of beyond to attain this...... The CAA would of had to attend those companies in the likes of the wilds of Scotland, and they simply do not have the staff to do that.
We original looked at having a Company on the next Airfield carry out our ARC issues and I would return the favour.. but was then informed that it would be ok for me to both Issue the ARC and carry out the work on the Aircraft as well, and as such our approvals are progressing with the system set up to do it this way in agreement with the CAA.....
So yes you can have one person responsible for both, after all is that not what we have already? the CAA previously issued the Certificate of Airworthiness based on the say so of the Nominated Engineer, who would possibly have also carried out the work required, without in 99% of cases ever seeing the Aircraft involved.
All the new system does is takes the CAA further out of the loop whilst further reducing their greatest fear....... Getting Sued