I have two subdomains on my site that have largely been independent of each other until we just started doing promotional content on one to direct to the other. Now I'm trying to get metrics on how much one feeds into the other. Each subdomain has its own GTM Container ID (within the same account).
I saw this post on how to track traffic between subdomains using the same Container ID. But I'm curious if it's possible to show subdomain traffic using different Container IDs, or if I need to create a new global Container ID at the top level that will be used across both sites.
I also use Universal Analytics, which all the subdomains share. So if it's not possible to do it with GTM, is this sort of thing possible with Universal Analytics?
And if I do need to make any changes with the existing configuration, will it only track traffic between subdomains going forward, or is there a way to do it to retroactively filter existing data so that I can still use the last month's worth of traffic data?