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:
Regular Licence: https://themeforest.net/licenses/terms/regular (Use, by you or one client, in a single end product which end users are not charged for)
Extended Licence: https://themeforest.net/licenses/terms/extended (Use, by you or one client, in a single end product which end users can be charged for)
You can however change the production and staging URL's at anytime within your Themeco account.
For guidance on updating the URL assigned to your licence code, please check out the following knowledgebase article:
I hope this helps.