ECP allows you to 'remotely mount' a database on one instance from another. For example, let's say you define (as you have) a Cache instance with a Database 'C' and you have a second instance running Ensemble now.
When the basic ECP configuration is done, you will then be able to define a 'remote' database on the Ensemble instance - the remote database being, the 'C' database managed by the Cache instance. The contents within the 'C' database on Cache will be available as a database on Ensemble just as though it was a locally mounted one.
In this scenario, the Cache instance is the ECP Server (or ECP Database server), and the Ensemble instance is the ECP Client (or ECP Application server instance).
The basic ECP configuration is carried our via ECP settings accessible through the management portal, and are done on the ECP client side (Ensemble side) - see Admin-> Settings -> ECP Settings. You need to tell Ensemble where the ECP Database Server (Cache) is.
See the documentation for details, but when that is done, your Cache databases will be available to Ensemble as databases that can be defined as remote databases.
Note: that ECP works at the database level - not at the namespace level... so the configuration of Ensemble Namespace (where you are building your production), needs to be enhanced such that the Ensemble namespaces sees the globals, packages and routines it needs. Edit the Ensemble namespace's Global, Package, and Routine mapping definitions.
Regarding SQL projections when you are on the ECP Client (Ensemble), if you do not Package Map your Class definition, then, you wont have those classes available for use. With only Global Mapping defined between your Ensemble namespace and the remotely mounted Cache database, you will only get direct global access.
The online documentation for ECP overview is here: http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=GDDM_overview
Finally, be aware that ECP is an enterprise feature, and that the corresponding license keys for your instances should include the Multi-Server feature. and be ECP capable.