This is not a fix but a way around, it works on my 64 bit system using 3 accounts at the same time.
First, add -shareArchive to your GW2 shortcut. Note that this option will prevent the game from being updated, so you’ll have to run the game normally every update.
Start the game using the shortcut and open Process Explorer (you need to download this, it's free) as an administrator. In Process Explorer, locate Gw2.exe and click on it. On the bottom half of the window is a list of items. Click the list and press M to scroll to the Mutant objects (sort the list by type). Right click the one named “AN-Mutex-Window-Guild Wars 2” and close it. Alternatively, you can press CTRL+F and simply search for it. You’ll need to close this handle every time you want to open a new client, since every client you open will recreate it.
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