Or, as the late Professor Joad was wont to say: "It all depends on what you mean by 'sole' ".
As an old stick-and-rudder man, I suppose I was always a 'sole' pilot in the sense that no one else was flying it at the time (apart from Guardian Angel, who grabbed the stick once or twice). Aircraft flown by two pilots at once do not, as a rule, do well. (Did not Sir Winston Churchill [who apparently did a bit] speak of "Stalling between two Fools ?")
D.