But there are also ways in which teaching two sections of the same class can keep one on one's toes. It's fun to see what two different groups of students will respond to in a given scene, and sometimes the readings go in interestingly different directions. It's also fun, in a way, to try to maintain the integrity of my lesson plan--which is to say, to make sure I hit a few of the same major points--in classes that may have wildly different interests or that move unevenly through the same material: one of my classes is always running long, which means I usually have to cut or summarize on the fly, while the other class often runs a little short and so gets spontaneous additional discussion. I like the puzzle-solving element of that: trying to keep both on the same reading schedule, covering the same big ideas, while responding to whatever arises organically in each one.
I've been doing this all year, and would have been flummoxed by it had I not been given the following piece of advice by both my old dissertation advisor and a professor at my current institution (both of whom, oddly, have firsthand knowledge of this). Teaching two sections of the same course back to back* is like have non-identical twins: no matter how much they might seem alike, and no matter how much you want to treat them exactly the same way, you have to recognize that they're different and have to be approached that way, even if they sometimes seem uncannily like each other.
*In my grad school days, I would TA two discussion sections, though never back-to-back, without this difficulty. I think in part it's that they weren't back to back, which heightens the oddness of repeating yourself or trying to avoid repeating yourself. It's also that the material was set for me from on high, i.e. the professor teaching the course, and so I was consequently less reflective on the pedagogical challenges involved. If it didn't go successfully in one class, I was liable to chalk that up to the professor's instructions.