Your scenario sounds perfectly do-able. However, I'm a bit uncertain as to which platform are you referring to? Alfresco ECM or Alfresco BPM (Activiti)? Either one of them should allow you to implement your process.
If you are running on Alfresco One and would like to implement this process on it, have a look at the documentation to see how to create an embedded workflow associated with a document Creating and managing workflows | Alfresco Documentation
Alfresco Activiti BPM
In the other hand, if you are using the BPM system which is the Alfresco Activiti BPM - Business Process Management | Alfresco, then you have all the flexibility of Java power to implement this kind of process. Your process should look like this
Also, have a look at Activiti user guide to understand how to set up task assignment for users Activiti User Guide
Let me know if you still feel uncertain as of how to achieve your process implementation goals.