Alot of my clients train with me twice a week which gives me a good amount of flexibility when designing their programs. Obviously two times a week is better than once a week since there is 100 percent more training time. However, there is another way of looking at a training schedule. Rather than focusing on how many times to exercise a week, look at how many workouts you have to get in a month.
Two sessions a week means eight sessions a month. If you have to cancel a session, rescheduling makes the most sense in order to get those eight workouts in. Most people just focus on the outcome and not the process of reaching a goal. For example, anyone can say I want to lose 10 pounds, but almost no one says I want to finish 32 workouts in four months to get those 10 pounds off. Having a process goal leads you to your outcome goal almost always.