I'm gonna call bullshit on that. I literally just shipped two ECUs back to a customer and the flash + MPC are IDENTICAL. You're welcome to receive the full backups of said ECUs.
EEPROM differences will be down to each ECU storing adaptive values for their prescribed functions.
Given that MED9 master and slave in all the SSPs is documented as wire selection, I'm inclined to lean towards my original statement being factual. The fact the VAG part numbers are identical is testament enough as there's not two different SGOs for master/slave.
I doubted, but you are right I looked at the master / slave and all files are the same! But I did find a Bosch e-mail that said there where problems with technicians swapping the plugs...Then the slave would have phantom misfires on the Slave only. My thoughts are that after installation they adapt, and can't be swapped after that.