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Note:
About the CK for the Special Edition, you can have several occurrences of the CK open at the same time by just setting the line below to 1 in the CreationKit.ini:
bAllowMultipleEditors=1
Things are simpler for this one since it's no longer hooked to Steam and you don't need to start the Bethesda.net launcher to open the editor.
I just did it. So here is how it works for me:
launch the CK in on line mode,
launch the Windows task manager, locate the Steam.exe *32 process, right click on it and kill it,
unplug your computer from the internet,
launch Steam and choose the off line mode,
launch the CK again; this time it will start,
re plug your computer to the internet, if you need it.
Launching two instances of the CK 64 bits requires that the line
bAllowMultipleEditor