Stream processing with join

[d4munche3z]

I am trying to create a stream process that can update at once from two stables. I want to do this as a join of the two stables so I can get the benefit of the data staying up to date and the efficiency of watermarking. However it seems like the streams can only aggregate from a single stable. How can I accomplish this? If I created an stable as a join of the two raw data tables the stable would not update in real-time as far as I know and If it could some how would it be efficient?