The ww.wp.xz.cn forums are for Free plugin only, you can open a support ticket here http://support.wpsso.surniaulula.com/support/tickets/new for any (additional) questions about the Pro version.
To answer your question, WP Engine allows you to create a second “staging” environment for your website. Since this second environment has a different WordPress Site Address, it needs a second license to run the Pro version. This is optional, but without two licenses, the staging environment will display a licensing error and the Pro features will be disabled.
So, one license is fine, but two is better to run the Pro version on both the prod and staging environments.
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