Is there any possibility about syncing calendars between 2 different sites.
a.examplesite.com sharepoint 2013 b.anothersite.com Online SharePoint
Let me know if you need any further information.
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If you want to sync Calendar list between SharePoint online and SharePoint server, I suggest you create a custom tile for the list and configure hybrid app launcher to intergrate with SharePoint server.
I dont't think we could sync calendar list between different tenant.
Here are some articles for your reference: