Good evening Ruth and welcome to the Events Calendar forum!
Thank you for your interest in our products.
We’ll be glad to answer your questions.
You will need 1 license per site for this to work properly.
The cheapest way to achieve this is to get a business license that lets you install the plugin on up to 3 sites while costing less than 2 licenses.
As a side note, if you are indeed using 2 identical sites, you might want to grab a copy of Event Aggregator: https://theeventscalendar.com/product/event-aggregator/
This lets you import events from another site automatically (which should save you a lot of entry time).
Best regards,
Geoff B.