I'm pretty sure it could have ended in one season and even been concluded in a way that went well, but to do that would mean throwing out a lot of the subplots along the way and focusing on the uber-plot. Too many of the episodes were filler and simply expanded on subplots that aren't important (or even related) to the main story, and I'm excluding the recap episodes.

I do admit being a little curious about how the story would have been wrapped up if the continuation/conclusion of Code Geass hadn't come in the form of a second TV season but as a 90-minute feature film instead.