Tracking user activity across two shopify domains

I was wondering if you could provide me any insight or point me in the right direction when it comes to enhancing user profile data. With the current shopify integration, user purchase data is available and so is browsing history etc...

What would be the best way for me to add in supplemental data for users from a separate shopify store front? For example I might want to be able to segment certain users based on their shopping patterns across two or more of our shopify domains instead of only the primary one that the tracking pixel is currently integrated with.

Is this something I would have to write a custom API integration for, or is it as simple as adding the same or a separate tracking pixel to the other shopify domain?

Is there a better way to approach solving this issue?