Usability testing is a bit of a luxury for a two-man team...
That said, we would pay a lot more attention to the on-boarding next game. I was quite happy with our tutorial so it's frustrating we left holes and fell short.
As Quentin said in another reply, one big mistake we made was that a lot of testing was with a beta group that had played the game dozens of times before the tutorial was fully fleshed out. Huge mistake! Another mistake was watching someone play it for the first time where we were in a position to coach them. We should've been silent and seen what they did. Easy to spot the issues in hindsight!
Check out the 'discount usability' movement. Much of it is written for web / mobile development but the principles are pretty much universal and can be applied to both game design and/or interactive learning aids (in your case the tutorial).
If nothing else read up on how to run a usability test. Without even being an expert it will help you get much more out of your testing.