Also related:
Options > Work Environment > More Options...
There is a checkbox for "
Launch new instance of ArchiCAD when opening a project from the recent list or by dragging onto a running ArchiCAD"
Mac OS X: With the above option checked, drag PLN files on top of the ARCHICAD dock icon and it will launch in a new instance of ARCHICAD or choose a project from the Recent list as mentioned and this will also launch the file in a new instance of ARCHICAD.
Windows: By default if you double-click a PLN file and then another, each subsequent file will be opened in a new instance of ARCHICAD or choose a project from the Recent list as mentioned and this will also launch the file in a new instance of ARCHICAD.
Best regards,