OK. The v12 engine is like two seperate engines using one crank.
It uses two:
fuel pumps
fuel filters
inlet manifolds
air mass meters
engine control units
throttle control units
electronic throttles
coils
So, it is getting spark and fuel.
Is the fuel pressure the same on both sides?
Should be about 2-3 bar
Are both throttle motors opening properly?
What is the flow rate of air on both sides according to the air mass meters? Are they the same?
Also what are the additve and multiply vaules according to the oxygen senors? This will indicate whether the faulty bank is running rich/lean.
This is a start.
You do need a very capable dianonsis machine for this, prefably a BMW one. If this is a "back street garage", I would strongly recc taking this to a BMW dealership. I know this is expensive but they should find the fault quicker.
Any questions, PM me.
Tristan