Hello Mark,
Data sources can't be separated between site collections, but you can choose specific calendar for a site page.
You can see how to do it here: https://www.virtosoftware.com/manual/calendar-app-for-sharepoint-online/choose-specific-calendar-for-a-site-page
Please find the pdf documentation in the attachment.
Mark Marriott
Am I missing something or is it not possible to run multiple instances that use different data sources? If I want to use a Calendar add-in on, say, another site in my Site Collection I can't remove the data sources that I don't need from my first calendar add-in - if I do they disappear from the calendar that I already created for another site.
The on-line guides are a little bit light on detail in many respects - and it woud be great if the on line help guides could also be in .pdf format.