To work on both drupal7 and drupal8 projects, we need drush6 and drush8 on our dev environment (in my case this is ubuntu 14.04 server). I'm doing this on a clean server with a lamp stack and git installed, no drush is present yet. This can also be used to install drush8 on a server with an existing drush version, in that case only use the commands for composer and drush8.
What this will do is use 'drush6' as the default drush command for 6 and 'drush8' as the default for 8. We will be using the terminal to install everything.