How to make sure a manager receives approval requests from certain users only
You can additionally limit access to Approvals by using tags. Tags are user-based labels that are used for data filtering across the application. For example, you can tag certain users with the tag "Approver Tom" and add this restriction to Tom's profile.
Go to Users > Tags to create a new tag.
Click on the button +Add tag at the top right corner.
Choose a name (e.g. Approver Tom) and choose a color.
Go to approver's profile and open Administrative restriction to assign tag Approver Tom into Approvals tag restrictions.
With these settings, Tom will only receive approval requests from users tagged with "approver-Tom" while still having access to the data of every user in the Sales department.
Setting up another manager for Tom
Setting up another manager for Tom is very similar. You would simply apply a new tag to Tom and then restrict another manager by the chosen tag. This would mean all of Tom's requests will be sent to the selected manager:
Any person with the Approval tag restriction set to "approver-Mike" will receive Tom's approval requests now. Setting up several such admins would send an approval request to all of them. The requests are then resolved on a first-come-first-serve basis.