Outlook has calendar sharing capabilities. Before taking time away from the office, the calendar owner can share a calendar with editing access to a designated person and can easily remove the access on return. Since you didn't mention which Outlook product you are using, I provided instructions for Outlook for the web and Outlook Exchange.
Outlook for the web:
Select Outlook Settings from the top menu then View All Outlook Settings.
Click on Calendar and Shared Calendar
@Select a calandar, select the proper you want to share
The Sharing and permission screen appears, enter the co-host's name and select Can Edit permission level
For Outlook Exchange,
Click on File from the top menu and Options
Click on the Calendar Tab and scroll to Calendar Options
Click on Free/Business Options
Click on the Permission tab, then the Add button to enter the co-host's name