There are many ways to teach Reiki and it is true that some teach Reiki I&II with a required time period between them and others teach them together. We allow teaching Reiki I&II both ways because his is how Hayashi Sensei one of the respected founders taught Reiki. When he was teaching at his school he taught them separately, but when he was traveling and teaching he taught Reiki I&II together in five days with about three hours of class time each day. See The Story of Dr. Chujiro Hayashi for more details on this topic including references. If you are a new teacher I suggest you teach them separately, but as you gain experience you can decide if you want to continue to teach them separately or if you’d like to teach them together. This is an individual decision.