In UEStudio open Advanced - Configuration - Application Layout - Miscellaneous and check the setting Allow multiple instances.
Now everytime uestudio.exe is called a new instance is opened.
Alternatively you can let the setting unchecked to open files with an extension associated to UES or files opened via Explorer context menu always in first instance of UES instead of opening every file in a new instance of UES. To open your projects nevertheless always in a new instance create shortcuts (*.lnk) with the command line:
"C:\Program Files\IDM Computer Solutions\UEStudio.exe" /fni "full name of the *.prj file"
for your projects. The command line parameter /fni which must be always the first parameter when used results in opening the project file and loading the workspace of the project in a new instance of UEStudio. The Force New Instance parameter is explained in help of UEStudio on page Command Line Parameters.
With using this method, I recommend unchecking the configuration setting Reload files previously open on startup at File Handling - Load so that calling UES without any project file as parameter does not result in reloading the project opened on exit of last instance of UES.
BTW: In menu Advanced there is also the command Open New Instance of UEStudio.