I've been researching how to add cross-domain tracking for two different domain names. We currently have two different properties for two different sites, Site A and Site B. Unfortunately we have a shared form system which lives on one subdomain (subdomain.sitea.com), but we share this form system across both websites.
The problem is, we can't follow the user's journey on Site B as, as soon as the user goes to Site's A form system it stops tracking the user's journey. It would also be nice to know if any visitors from Site A leave, but visit Site B at another time - basically giving us an idea of shared visitors.
I've been reading a lot about cross-domain tracking with GA, but it sounds like it only works if both sites are on the same property in Google Analytics. I don't want to lose any of the historic data relating to either sites either.
Is it possible to create another property and load the new GA tag on both sites, effectively loading two GA tags on both domains? Or is there another way of tackling this problem without losing our Google Analytics data?