Flying Crane
Sr. Grandmaster
If the one system was specifically developed to rely on the other, then that makes sense.One of my sensei's trained in Fu Jow Pai. I learned two of the forms from either fu jow pai or hung gar (can't recall which, although can still perform the forms) from him, as a way for my movements not to be so 'hard', but didn't actually learn either system. I would ask him questions, and he explained to me that system works the same way you described/are doubtful of...you have to learn hung gar before you learn fu jow pai. According to him, fu jow pai was based around hung gar and focused on one specific aspect, so you had to learn the system as a whole before specializing. It wasn't that they were developed hand in hand, but rather that learning the second system would be incomplete without first learning the first system. Makes sense to me.
If it was developed independently, then it would not make sense.