Run either command below in Terminal while one Astah is already open.
To launch another Astah:
open -n /Applications/astah\ professional/astah\ professional.app
(Let it point to your Astah app file)
* The text in orange will be replaced with the edition you have. (sysml, community or gsn)
To launch another Astah with a specific file opened:
open -n xxx.asta (eg: open -n /Users/xxxx/Desktop/Class.asta)