Here we are again, revisiting an old theme. When using Google Tag Manager, we often want to send the contents of the same tag to multiple Universal Analytics properties. With on-page GA, this used to be quite simple, as all you had to do was create a new tracker and then just remember to run the ga('trackerName.send'...) commands to all the trackers (or you could use my duplicator plugin). With GTM, your options are more limited, since Google Tag Manager abstracts the tracker object, giving you far fewer tools to work with.
One of the things I’ve recommended from the get-go is to always send the Client ID to Google Analytics with your users’ hits. This is very useful for adding a level of granularity to your tracking. At first, I recommended using an Event tag to do this. Then I modified my approach a little so that you could send it with your initial Page View (thus not inflating your hit counts).