You will want to create 1 export url on the main site. You can place it in a hidden menu item if you like so it is not easily accessible.
Create one called Export JEvents
Then, access if via your website and select what you want to export, click export. Then underneath the button a link should appear right click it and click copy link.
Now go to your other JEvents sites and click JEvents and Calendars, then click new and import via url.
Once setup you will want to create a cron job to automatically import the events.
You might find this useful:http://www.jevents.net/jevents-15-topme ... -and-icals