Starting Process from another

To explain fully:

Both processes will be running at exactly the same time becase they are sensitive to any event on the clock or reset. The first process will schedule "start_S" to change at the end of the current delta cycle, but it remain what is was for the this delta cycle, and as the other process is running at exactly the same time, it see start_S without change.

Now, if start_S was a variable or a shared variable, it really would be important when it was updated, and in the case of a shared variable, you'd have no idea whether it had changed or not by the time you were trying to read it. But you shouldnt worry about that because you shouldnt be using shared variables outside of testbenches anyway.