After you have divided your employees into organizational units and setup some managers for them, you will probably want to setup managers of your managers.
Limiting admin access
Limiting an Admin or Manager to a specific org. unit will limit their access to that unit only. They will also receive requests from that org. unit only. This can be done on the user profile:
Making sure a manager receives Approval requests from certain users only
You can additionally limit access to Approvals by using tags. For example, you can tag certain users with a tag "approver-Steve" and add this restriction to Steve's profile:
With these settings, Steve will only receive approval requests from users tagged with "approver-Steve" while still having access to data of every user in the Hardware Marketing org. unit.
Setting up another manager for Steve
Setting up another manager for Steve is very similar. You would simply apply a new tag to Steve and then restrict another manager by the chosen tag. This would mean all of Steve's requests will be sent to the selected manager:
Any person with the Approval tag restriction set to "approver-Emily" will receive Steve's approval requests now. Setting up several such admins would send an approval request to all of them. The requests is then resolved on a first-come-first-serve basis.