You can reduce your blind spots to smaller than a motorcycle or remove them entirely depending on mirror size: https://www.caranddriver.com/features/a15131074/how-to-adjus...
I’ve been using this method since before this article. I still look, but there’s never a car when I look.
This, everyone should adjust their mirrors that way and work at getting used to it. But I still look anyway like you do, and like you there was never a surprise car there until a few weeks ago and suddenly there was: A vette going a lot faster than my 75MPH and crossing from one rear quarter to the other then flying ahead as I was preparing to pass the car ahead of me from the center lane. I noticed it only because both eyes over my shoulder caught his flight across the periphery of my gaze.
You're free to use both (I do), but you only need (and are required) to look behind to ensure there isn't a car in the blind spot or just behind it.
There's a reason why in many states you need only one side view mirror. In some states, merely having the rear view mirror suffices (i.e. no side view mirrors required).