There wasn't a co-pilots' solo when I went through the OCU in the early 90s, I'm surprised that it ever happened as the co was only trained to sit in the RHS. There was a crew solo where we went off without any of the instructors but it was made absolutely clear that the student captain was to be the handling pilot, I was not to touch the controls. By the time I did my captain's course that sortie had been taken out of the syllabus.