These days certain systems and components are deemed "Safety Critical" and must be maintained by different people or on different occasions.
ie. Oil, Fuel and Hydraulic filter maintenance, Boroscope inspection ports, starter and generator replacement, etc.
This way, if an error is made it will only affect one engine or system.
A little while ago I had a task to replace the wing front spar main attach bolts on a DHC8-Q400. This was listed as a "Safety Critical" task. Now these bolts are very important, but if one side was cocked up and the wing came off, the other side would be pretty useless on its own. We had a smile at that idea.