Definitely both at the same time... Although that's not an option on the survey.
I've heard it said that if you are seeking sex with someone new, swap hands.If you use your subdominant hand it often has the feeling that someone else is doing the job. This is sensed by both your hand and your dick.
That feeling might be explained as a result of Bicameralism, a theory that supports the idea that one side of the brain may not fully know what the other side is doing.
It is a theory being played with to some degree in the new HBO 2018 Westworld TV series.
I probably favored my right hand most of the time, as I am right-handed, but I'm sure I switched off, and switched back and forth.
At one point in my life, I wondered if the slight curve to the left in my erection was caused by so much use of the right hand. As an experiment, I started using my left hand almost all of the time. Over time I noticed that the slight left curve had gotten even slighter, and eventually was non-existent.
... and since I can, sometimes I use both hands at the same time! I'm ambidickstrous!