By default the Mac OsX does not allow the user to run multiple sessions of the same application but with the following workaround you can overcome this limitation for the Trader Workstation application.
you can then use this procedure which is the most simple:
1) Open the web page www.ibkr.com
2) Go to the menu Technology --> TWS Software.
3) First download and install one of the auto-updating TWS versions (TWS LATEST or TWS according to your preference).
4) Once you have completed the installation, download and install one of the offline versions (OFFLINE TWS LATEST or OFFLINE TWS according to your preference).
5) You will then see on your Desktop two different Trader Workstation icons. You can use the first shortcut to start the first TWS session and the second one to start the additional one, as you can see on the screenshot below:
1) Launch the first session of the Trader Workstation using the TWS icon present on your Desktop. If you still do not have any Trader Workstation icon, please go to the web page www.ibkr.com and click on the menu Technology -> TWS Software. Then download and install one of the TWS versions present there.
2) Open the Finder and click on Applications on the left side menu. Then select Utilities and then launch Terminal.
3) The Terminal will appear on the screen and will point to the home folder of your current Mac OsX user. Now please enter the commands listed below (as well displayed on the screenshot) on your Terminal. After each one, press ENTER on your keyboard.
cd Applications
cd Trader\ Workstation
open -n Trader\ Workstation.app
4) The login window for your second TWS session will then appear on your Desktop. You can now close the Terminal.
Note: the procedure above presumes that the TWS executable is located in the following path: users/yourusername/home/Applications/Trader Workstation/Trader Workstation.app
If at the time you installed the TWS, you selected to install it for all the users of your system, then the TWS executable will be located here:
/Applications/Trader Workstation/Trader Workstation.app
and therefore you would need to issue the following commands in the Terminal:
cd /Applications
open -n Trader\ Workstation.app