Merge the Calendar with another site
From EdTech Help
If you would like to combine calendar items from two or more sites, you may choose to merge the calendars.
Remember that all calendar entries for sites you have access to are automatically merged in your Home Calendar.
Note: You must have appropriate level permissions (i.e. calendar owner) to merge calendars. Merged calendar items will only be visible for site participants who are active in both sites.
Go to Calendar.
Select the Calendar tool from the Tool Menu in your site.
Click Merge Internal Calendars
Select calendars to be merged
In the Show Schedule column, check the box next to any calendars from other sites that you would like to merge with the current site.