In Boomi, you have two sub-processes that are configured to run asynchronously. If you plan to share the value of the sub-processes in this mode, you will need to be aware of the behaviour of the process property as it is not feasible.
For example, in Sub Process 1, you have a Set Properties shape that will set a value to a process property, and you want to get the value of the same process property in Sub Process 2. At this point, Boomi will return you a null value in Sub Process 2. This is because both of the sub-processes execution are considered running in different threads.
If you want to share the value of a process property between processes, you can configure the process to run synchronously.