Plug two headphones on a MBP 2011 15" Is there a way to use Mic/Line in as an output to plug a second set of earphones? 

I gave a shot with JackPilot which is not compatible with macOS 10.12.3.
No, however you could purchase a USB device that gives you a headphone output and use Audio MIDI Setup in macOS to aggregate outputs so it would output on both. Steps for that are here:

http://lifehacker.com/5933282/how-to-aggregate-and-use-multiple-audio-inputs-and-outputs-in-os-x

Alternatively, just buy a headphone splitter to split your one 3.5 mm jack into two. Like this:

https://www.amazon.com/Belkin-Speaker-and-Headphone-Splitter/dp/B00009WQSR