You would only be able to use the X Theme on one site at one time, but you could also specify a staging URL which allows you to have two X Themes on the go at once under one licence so you’d have the main production site i.e. www.mydomain.com and a staging site i.e. www.staging.mydomain.com
This is in accordance to the terms of service set out by Envato with their Regular and Extended Licences: