I took both of these levels and had him do a lesson from each a couple of different times to determine how they differed, and how they could be used together. They can be used together with great ease! The Level 3 has more emphasis on writing, while the Level 2, for me anyway, emphasized oral narration to a greater degree. They both have the same questions, and the same lessons each day. So if you are on the borderline, like we are, either level will work just fine.
With that said, once I figured this out, We did one lesson per day. On some days we did more writing with Level 3, on other days we stayed with Level 2 for more oral reciting. One thing I really, really like is the section review! The section review is comprised of a list of questions that go over what you have learned in the previous section. In the review, the levels differ a bit, but not so much that you can't use them together. For example, in the 1st section review, Level 2 had 3 additional questions that were not in Level 3.