It's that quality versus quantity thing again. I take one joint lesson with my partner per week, and we practice for an hour afterwards. Then we practice two days later, and then again two days before the next lesson. Sometimes on of us has an off day, and sometimes we both do, but we seem to sense this in ourselves, communicate to eachother, and try to stay understanding of one another. Also, because we don't practice every day, we tend to focus on maximizing the time we have for practice.