Not sure where to start but here goes.
1. Create both the user ID
2. Create the folder
3. Right Click on the folder and select Properties
4. IF you are sharing the folder, Click on Share this folder.
5. At Permission select the 2 user id to access the folder. Remove “Everyone” from the folder.
6. At the next tab which is security, click Advance.
7. Untick on Allow Inheritable permissions from the parent to propogate etc etc
8. Click Remove
9. Add back the 2 user ID that can access the folder and put the user accessible rights to the folder.
10. Click Apply all the way and test it if it can work.
Your domain admin would still be able to add himself with rights to the file. Password protect the file or use 3rd party software to protect the folder the file is in. And think about the trust level you have with the domain admins. No job is worth a peek at a bonus spreadsheet or payroll ledger or other confidential document.
The backup administrators will need to have access to this volume so they can be able to backup and restore the contents. Domain admins can gain access to the file with overwrites. For your peace of mind, you can install a product like Varonis http://www.varonis.com/products/datadvantage/windows/ which will give you “eyes” onto the contents. We use this in our environment which includes highly confidential information being part of a pharmaceutical company.