In response to the question posed earlier about simultaneous maintenance of engines,it is now standard practice to only carry out maintenance to 'critical' systems,such as disturbing oil or fuel lines,on one engine [for a twin] at a time.Any disturbance,such as gearbox turning cover etc during a borescope insp requires a duplicate inspection.
However,it is acceptable,if needs must,to carry out maintenance to 'critical' systems on both engines at the same time if the individuals reverse their roles ie. who ever does the initial inspection on one engine does the duplicate insp on the other one and vice-versa.