You can install Site.pro builder plugin on multiple hosting panels (e.g., Plesk, cPanel, ISPmanager, DirectAdmin) with a single license (free and paid).
To use Site.pro plugin on cPanel and Plesk servers mixed, you should install builder plugin on both panels by following instructions on your brand.
Open your brand, go to "Install plugin" section and click button "Install plugin". Choose needed panel (e.g., cPanel or WHMCS) on the dropdown to get plugin installation instructions.