@Anonymous
You have asked what happens when one approver approves and the other rejects. That depends entirely on how you Flow is constructed and how the approval is configured. Th easiest way to approach this is to set a single approval action to use a Start and wait for an approval action and the All must approve option. If either approver rejects then the approval outcome is reject.
If the department manager is also the requestors manager, the you can use the Office 365 Users Get Manager action to retrieve the information required to use them as an approver. If not, then an easy way to approach this would be to have a separate list of departments and managers. Then lookup the manager before assigning the approvers.
You'll likely want to assign the department manager to a compose action and reference the compose action when assigning approvers.
After the approval has completed, send an email to the items creator and Accounting if the Outcome is Approve and only to the creator is the Outcome is other than Approve - use a condition.
My suggestion for you is to give this a try and post a screen shot of your Flow along with any errors if you have a question.