The update via openvas -u should not take 60 minutes. I suppose there is something wrong with your setup. For me a clean full load of all VTs into redis takes between 5 to 10 minutes.
But to answer your question about using the same redis instance for sharing data between several openvas scanners I would not recommend doing that. It might work in some cases but there is actually no code that ensures concurrent access of several openvas or ospd-openvas processes. Both services assume that only one instance of them is running at the same time. It will break and fail one way or another.