This sounds similar to something I'm doing, and have had a lot of issues with, due to the approval process no longer being in SharePoint Designer 2013.
However, your process is certainly possible doing something similar to the following:
Create a 2010 workflow in SharePoint Designer 2013 to handle the end of the approval process. You can use the Content Approval task in a 2010 workflow. This will deal with publishing your document and making it visible to everyone.
Create a 2013 workflow in SharePoint Designer 2013. In this workflow, you can "Start a Task Process", which will allow you to assign a task to more than one user (user A and user B), and choose the drop down for "Serial", as opposed to "Parallel". This will ensure user A needs to approve the item before user B is assigned a task to approve.
If user B then approves the item as well, your 2010 workflow (created earlier) can be called from within your 2013 workflow to do the content approval and publishing of the document. You use the action "Start a List Workflow" to accomplish this.
To give an indication in stages:
Stage One - Check Status
Check the status of the document to ensure it's at draft (or whichever status you need) and assign this to a variable.
If the status variable is OK, go to stage two.
If it isn't, end the workflow.
Stage Two - Document Approval Task
Start a Task Process with users A and B in Serial. Ask them to review the document. Store the approval variable in Outcome.
If Outcome is Approved, go to stage three.
If it isn't, end the workflow.
Stage Three - Publish the Document
Start a List Workflow with your 2010 content approval workflow.
Go to "End of Workflow".
This is just a simple example, but I hope it helps you.