Make one MacBook Pro use the screen of another MacBook Pro I have two MacBook Pros: a 2011 17 inch  and a 2017 15 inch; both have the latest macOS installed. 

Does anyone know if it is possible to make the 15" MBP  use the 17" MBP as an external monitor?
You can't physically connect the display of another MacBook (or any other laptop for that matter) to use it as an external monitor.

However, there is an app called AirDisplay that will turn any display into an external monitor via WiFi (or wired network if you prefer).  


  Wirelessly connect to an iPad or Android device from your laptop to
  create the ultimate road-warrior workstation. Or repurpose any nearby
  Mac or Windows PC to give yourself more screen real estate for your
  main Mac.


It's a pretty good piece of kit, but (IMO) it's better to just purchase an external monitor that meets your needs; it's much more stable, reliable, and better performing.