I couldn't see the Run Multiple Instances of Band in a Box in the Mac version settings (maybe it's hidden some where ?)

So I just used:

This will start Baib from the shortcut allowing you to run multiple instances.
You may have to right click > Open to run the first time.

https://www.dropbox.com/s/pd55njtmor1fbvv/Biab.app.zip?dl=1
Click No Thanks Continue at bottom to skip login

To create it manually:
In Applications/Utilities open AppleScript Editor.app

Copy & Paste this whole line including "

do shell script "open -n "

into the window

now drag the band-in-a-Box.app (or any other you want multiples of)
to the left of the last " so you have

do shell script "open -n /Applications/Band-in-a-Box/Band-in-a-Box.app"

now go to the App's Menu > Script > Compile (or Command-K)

now Save and change the file type from script to application

now if you right click on the new app shortcut > get info

now drag the original Band-in-a-Box.app onto the icon at the top of the info window to create an icon for it.