Wannabe trainee pilots on the RAAF's No 1 post WW2 training course all went solo with another trainee in the R seat before being granted wings. Must have ben a bit risky as we didn't know much about Vmca and other finer points. MUCH later managed to get one off the ground on one engine, but required 35 kts of wind to enable and a large semi-circle ground run.
Then how many Gooney drivers know that all of these old ladies will ignore your control inputs if both pilots wind in full opposite aileron at the same time.
Wasn't game to try the same in pitch but I think the designed stretchiness in the control cables would be similar in pitch.