I found it useful to have multiple Stata instances running on my Mac, in particular, if I use one instance to clean the data before running merge commands. It is always annoying if the merging does not work out or throws an error and then, one would have to clear the current instance and open the DTA file that was messing up the merge.
Its a simple command that allows you to open multiple Stata instances on a Mac:
open -n /Applications/Stata12_OSX/StataSE.app
You can also define an alias command in your .bash_profile,
alias stata='open -n /Applications/Stata12_OSX/StataSE.app'