Multiple venues for a single event

The premium version allows you to create multiple instances of the same event called a Datetime. You can sell tickets to individual datetimes or you can sell tickets that allow access to all datetimes.

https://eventespresso.com/features/multiple-dates-times-per-event/

The congress we are organizing will be held simultaneously in two locations (in different cities), so I don’t need multiple datetimes for the (single) event, but multiple locations.

You can input both venues into the Event Espresso > Venues admin. Only one of the venues can be selected in the event editor though. What you can do is include information about (or links to) both venues on the event page.