Usually when I shoot with a second shooter, I tell them exactly what I expect them to shoot and how I want them to shoot... My last wedding, amateur, good but not great photographer... He shot 7d and 5d2 and used 135L and 24-105.... It was slightly low light and 90-95% of his 5d2 shots were unusable and he had a slightly better keeper rate with the 7d... Long story short, despite telling him exactly what I wanted, I shot similar stuff just in case, and for the most part, I ended up using only a handful of his shots and most of mine... It was a great learning experience for him and I sat down with him afterwards and critiqued his shooting with him no bars held... He learned quickly... the second shoot I had with him, it was an outdoors event and better lighting, but his keeper rate was much much better... In the end, gear can be crucial, but also you much teach your second shooters, talk to them, and guide them... If they dont know why what they did was wrong or doesn't work, then they would never know and wont improve...