I don’t think the best solution is sharing data between two Java Actions. Each Java Action has it’s own scope, so I think it’s better to pass the data from your microflow to your Java Action. The same for your other Java Action: pass the data from your microflow to your Java Action.
Even though this is not the first approach you should choose, you can attach data to the current context:
This data will be available within the current execution context (e.g. a single web request from the client).
Can you make sure the output of the Java Action has exactly the information you need to pass from the one action to the other? You would need to create interface objects in Mendix for this output, but then you can keep it lean and only pass the specific information needed.