Kimoki its a good question.
It is an oddity that on ETOPS aircraft, specifically, and multi engined aircraft in general, it is good maintenance practice not to work on more than one powerplant at any one time (safty critical task in the MPD).
Software upgrades to the FADEC is an exception to this as you normally cannot operate the aircraft with a different software configuration installed on each engine i.e. update one engine, fly and then update the other. Most times you don't even need to perform an engine run after the update. The FADEC software is categorised as "critical" but on other systems errors do creep into the lines of code which is why we end up with FMS version 10.6, 10.7, etc, which is not always to add functionality but more often to correct mistakes...