I had two trainers for a while! We boarded our horses at two different stables due to room/training reasons (one was at a training barn, the other we just boarded). I learned under each of the trainers at either barn. One was a lot like your trainer, very methodical and waited until I mastered the skill to progress. The other was a different story. She taught me the basics, taught me more stuff and different ways to accomplish them and then began to teach me different disciplines to improve my riding overall. I rode western with the methodical, slow one all the time and so when I went to the other trainer she eventually left it up to me to decide. I could go from riding western in both lessons to riding western and then english, or western and then driving a carriage. But no matter what the other trainer always made sure to relate it back to improving the overall horse skills and keeping everything safe and without any 'holes'. That being said she did take her time in her own way. Since I was riding with another trainer she expected me to constantly work on the skills we had practiced with her and when we were in the lesson she expected me to be dilligent and an able rider. If I slacked off she pushed me 10X harder and wouldn't let me off the horse till I AND the horse ended on a good note.
I think it's good to have multiple trainers because it gives you many viewpoints, different options on how to handle a horse, and overall improve riding by riding different horses and learning different ways. I haven't met a trainer that minded severely (the two I worked with didn't) and I don't see any reason why yours should. To put it in a kinda harsh way, business is business right? I think as long as she knows your not doing it because your upset or completely leaving because of an issue, but rather to gain more knowledge she should be fine. Just be honest and upfront about what you want to do, and if you do see another trainer, what you want to know/expand upon. :)