Here are the steps:
User1 wants to see User2's mailbox.
1. Open User2's mailbox from Office Outlook.
2. Right click on the Inbox folder and click on Properties.
3. Once there click on Permissions tab:
4. Click on Add and select the user2's username.
5. Assign appropriate permission level to that user as shown in the image above.
6. Open user2's Outlook Web Access(OWA) account by going to outlook.com/yourdomain.com
7. Right click on the root folder and click on Open Other User's Inbox:
8. Enter that user's name and click on Ok.
9. You will see User1's inbox (separate folder) in your Account/OWA.