How do I track the same visitor across multiple domains?
Cross-domain tracking is very easily set up with Piwik PRO Analytics Suite.
First of all, you need to make sure that all of your domains are added to your website settings.
To check this click left upper corner Menu and select Administration from the drop-down menu. Choose the Websites & apps. Locate your website and fill your domains in the URLs section
Now open your Tag Manager, choose your Piwik PRO tag and switch the Enable cross-domain tracking button
That’s it, your visitors will now pass an additional query parameter when moving between your domains. That parameter will be used by Piwik to identify your visitors.