How to Install multiple version of Drush?
In your .bashrc:

alias drush5=/path/to/drush5/drush
alias drush6=/path/to/drush6/drush


With this, drush5 will give you Drush 5, drush6 will give you Drush 6, and drush will give you whichever of these appears first on your $PATH.

Rename your aliases to suit.