Well, usually I write while my copilot flies, or vice versa.
However, it's completely feasible to write & fly, especially in a turbine. One of the first things I learned in the TH55 was to fly with my knees, while lighting a cigarette with gopher matches, which required the use of both hands. If I need to fly & write, I either fly with my left hand or with my knees, while I write, but a decent AFCS makes it easier. I keep my kneeboard on my left leg anyway, so it's not under my arm while I'm flying with my arm on my right knee/thigh.