What is the action of writing with both hands simultaneously called?

I need the word that means simultaneously using both the hands, for writing or whatever.

Actions involving both hands simultaneously are called bimanual. Playing a piano is an example of bimanual activity. Others would be tying shoes, clapping hands, etc. As @ Third News commented above, researchers use the term bimanual simultaneous handwriting, but I wouldn't expect that to be in common parlance. The problem with seeking a single word for an obscure condition or rare phenonmenon is that even if if you find one, it is likely to be of little use since no one would understand it without the definition being provided. Still, how would you know if you don't ask.

There are clever and high-falutin' ways to convey the idea of writing with both hands simultaneously but employing the K.I.S.S. approach - Keep It Simple, Stupid! :-), why not just say that "he/she wrote with both hands simultaneously"?

To me, it is obvious that someone who is writing with both hands, simultaneously or alternatively, is ambidextrous so that word isn't really needed, is it?

Synamphichiry would be properly formed from Greek, meaning something like "using both hands together at the same time" (syn- expressing "together at the same time").

Amphichirography would mean "writing with both hands".