Mailplaner is a project mainly written in Shell, it's free.
Create a copy of Mailplane.app
Read as "Mail Plane-er", it creates a copy of the Mailplane.app executable so
that you can run two copies at once (for instance, one for your work account
and one for your personal account). You can also create a custom icon, call it
Mailplane.icns, and it will replace the icon when copying.
Make sure to turn off auto-updating in your copy, and re-run the script when your main copy gets an update.
Change the SUFFIX= in mailplaner.sh to the suffix you want and run it!
./mailplaner.sh