Best bet is to follow [the documentation][1] on federating an ArcGIS Server site (which can be comprised of multiple GIS Server nodes), as these provide more detail.

Here is the high level workflow.  Below screenshot is from Portal for ArcGIS Administrator settings, and the Portal has already been federated against one ArcGIS Site. 

[![enter image description here][2]][2]

To add another site:

 1. Hit Servers tab

 2. Add an ArcGIS Server Site.  Point to the REST services URL, the Admin URL, and provide a username and password of the primary site administrator (default username is siteadmin).

 3. Choose which ArcGIS Server Site will be [the hosting server][3].

You should now see the services from the second ArcGIS Server Site appear as new items in your Portal.


  [1]: http://server.arcgis.com/en/portal/latest/administer/linux/federate-an-arcgis-server-site-with-your-portal.htm
  [2]: https://i.sstatic.net/R6FK5.png
  [3]: http://server.arcgis.com/en/portal/latest/administer/linux/configure-hosting-server-for-portal.htm