Thank you for using the Communities to ask this very interesting quesiton!
If you have two analyzers with a 120 WatchPoint (WP) license each, you'll get 240 WP total.
Child WP concept only exists on APM Console. But watch out: on the APM Console, you'll only have application WPs.
The application WP depends on the Application Name Custom Field, which by default extracts the URI hostname.
The child WP on the APM Console will allow you to add more grammar to the filter defining the application WP you choose.
It means that if you have WP "foo" on analyzer-1 and WP "bar" on analyzer-2, you will not be adding "bar" as a child WP of "foo" on your APMC.
If "foo.example.com" is an aplication monitored by EUEM, then you will be able to monitor it in your APM Console as an application WP, and then you will have the possibilty to add child WPs to it.
The APMConsole sees a total number of watchpoints from all the analyzers that are loaded as components. Regardless if you configure a Application watchpoint or a child watchpoint - the cost is still a watchpoint. A child watchpoint will gather metrics based of a subset of traffic configured by the application watchpoint
To recap - if you have 2 Analyzers, 120 watchpoints each, you have a total of 240 watchpoints you can configure. You can configure regular watchpoints on the analyzers or application/child watchpoints on the APMConsole and each one subtractions from the total amount of 240.