GA: Custom Integration Tips
Once configured, test the cross-domain tracking setup by navigating from one domain to another, and checking Google Analytics to make sure you are counted as one user with one session, not one user with two sessions. We start with Real-Time reporting, and use the Google URL builder to generate a unique Source/Medium so that we can easily distinguish our test sessions in the reports.
For each of the two instances of Google Analytics tracking, make sure you set the cookies to where one will not overwrite the other. Otherwise, you will never see session data load into both properties.
Create a new Medium called “Offline” when loading those sessions into Google Analytics, and apply that to your data sets via Google’s Measurement Protocol. This will allow you to isolate that data in the reports. Also, it makes the Multi-Channel reporting so much more valuable, when you are able to view and filter strings of website sessions that lead to an offline transaction.