Yes, that is possible. Exit all running instances of UltraEdit except one and uncheck configuration setting Allow multiple instances.

All files opened by double click are opened by default now in first running instance of UltraEdit.

If you want one more instance, you can use Advanced - Open New Instance of UltraEdit.

To close a file in first instance and re-open it in a new instance you can left click on file tab of the file to move to a new instance, hold left mouse button and drag the file tab (mouse pointer displays as text page) to somewhere outside of UltraEdit main window not occupied by another application window. This method of moving a file to a new instance of UltraEdit works of course only if whether UltraEdit nor any other application window is maximized as otherwise you will not find a free desktop area where you can drop the file tab for closing file in current instance of UE, starting a new instance of UE and open the file there.

It is of course also possible to move file tabs with drag and drop from one UE instance to another already running instance.

If you use a project which you always want to start in a new instance, create a shortcut (*.lnk) for this project with the command line:

Windows x86: "%ProgramFiles%\IDM Computer Solutions\UltraEdit\uedit32.exe" /fni "Full path and name of *.prj file of project"

Windows x64: "%ProgramFiles(x86)%\IDM Computer Solutions\UltraEdit\uedit32.exe" /fni "Full path and name of *.prj file of project"

/fni which must be first parameter means: force new instance

The configuration setting Files selected in Explorer will open in a new single instance could be also useful.