Would you please provide us more details? What entity you are using for Sales Rep and what you are using for Sales Manager? Are you asking about Organization and Contacts relationship? Or you are using a custom entity for sales rep and manager?
We are using native sales rep and sales manager functionality. We can assign one sales manager to one sales rep but in our business that is not accurate. We have one sales rep and can have multiple sales managers helping with the accounts of a sales rep.
We have a few sales managers that need to be assigned to multiple sales reps. Example Sales Manager A is assigned to Sales Rep 10 and Sales Manager A also needs to be a sales manager for certain customers of Sales Rep 4 and Sales Rep 5. The biggest downfall is that Sales Manager A can only see customers for Sales Rep 10 but he is on a project that he needs to see more customers than just for Sales Rep 10.