The utag_main_ is a first party cookie, it is loaded on your firstdomain. Once on the seconddomain this cookie is a 'third party' cookie.
Third party cookies are also placed by advertisers and analytics, when you request a resource directly from an advertiser, like a pixel, it comes from a server on a different domain and it can place a 'cookie' when you make that request. Tealium's files are served from a CDN network on 'another' server but we do not place the cookies directly when serving those files, rather the code is loaded and run from within your code and off your webserver's domain. Hence the first party cookie.
TIQ does not have a built in third party cookie sync or matching service baked into the code. You can check with your specific analytics or advertiser vendor and see if they provide a cookie sync or matching service. You can then load that solution in tealium to process the cookies with their server/cookie/sync service.
Some other possible solutions would be to do some custom javascript to pass a query string for outbound links, that then pass the ID to the new domain... or if you have AudienceStream, then we automatically keep a 3rd party ID across domains.
If you have access to dev resources on your sites, then you can use a combination of placing a file on both webservers/domains and then loading that file from the 'other' domain in an iframe with the values getting passed in a query parameter. The 'other' domain requests this specific file from each other through an iframe. This file on the webserver would be blank but loading a specific utag.js file that reads the query parameters and sets them into a cookie. Since this file is requested off a server in another domain, it then writes the cookie on the other domain ( third party cookie ) . When you navigate to the second domain this same cookie becomes a first party cookie and can then be read. *source = @jason_paddock
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