This can be handy if you don't have multiple physical drives from which to run each instance or if you have a speedy drive (ssd or ramdrive or whatever) where storage space is an issue.
Try at your own risk, some understanding of how Windows works required, if something blows up my plan has met with success, and, no, you can't log in multiple characters from the same account simultaneously (though that would be a good vip perk).
Step 1 - Make a symbolic directory pointing to your DDO installation directory.
Windows 7 Pro 64bit as administrator example:
Use the Windows mklink utility to create the symbolic link.
mklink /D \fakeDirectory \real\DDO\Directory
on my machine, it looks like this:
mklink /D c:\fakeDDO "c:\Program Files (x86)\Turbine\Dungeons & Dragons Online - Stormreach\"
Windows XP Pro example:
Download the junction utility from microsoft if you don't already have it.
junction \fakeDirectory \realDDO\Directory
on my old machine, it looks like this:
junction c:\fakeDDO "e:\Program Files\Turbine\Dungeons & Dragons Online - Stormreach\"
Step 2 - Create a shortcut pointing to turbineinvoker.exe in the "new" directory.
Step 3 - Start up DDO using both the original shortcut and the new shortcut.
Step 4 - Log in the first account, alt-tab, and log in the second account.
If anything patched, let your original install finish patching before hitting the new shortcut.
On the launcher, if you click the down arrow, select Options, pick the Advanced tab, and check the "Show the launcher in the system tray" box, you will be able to log back in an account that is logged out/timed out/memory leak crashed without being required to logout all accounts first.
You can run more than two instances at a time (just make more symbolic directories and shortcuts with different names), but keep in mind each one will need a gig or so of ram to play with.
Just as a final example, here are the contents of the .bat file I use to fire up DDO; sets up a ramdrive, copies the ddo directory from my hard drive to the ramdrive, creates a symbolic link to the ddo directory on the ramdrive, and fires up two instances of DDO:
imdisk -a -s 10G -m F: -p "/fs:ntfs /q /y"
xcopy "c:\Program Files (x86)\Turbine\*" f:\ /E
mklink /D f:\fakeDDO\ "f:\Dungeons & Dragons Online - Stormreach\"
"f:\Dungeons & Dragons Online - Stormreach\turbineinvoker.exe"
Edit: I hadn't meant for this to be a tutorial on setting up a ramdisk, but, oh well. You can find imdisk (freeware tool that can, among other things, make a ramdisk) here:
Take a peek at the total size of your ddo directory (plus a bit for screenshots or future patches), toss in a gig or so for each instance of DDO you want to run, and another gig for the devil's cut, and that's at least how much ram you'll need. In my case, my directory size is 8.1G, so rounded up to 9G for the size of the ramdisk. Toss in 2G for each of two instances of DDO, and another gig for Windows, and I need a total of 12G of ram. Deathsapprentice down in post 9 has some excellent insights on how to go about reducing the ram required.
@post 12: Quotation marks around files or directories are required when a command line argument is made with non-DOS standard characters in a file or directory name (such as a space or ampersand).
@post 15: Running two instances of DDO with only 2G of ram would require a lot of virtual memory; while it should work, it wouldn't be super. If it worked with two installs, it will work with one and a symbolic link also.
U14 Edit: Increased example ramdisk size to accomodate u14 stuff.
U15 Edit: ^ that again
U17 Edit: ^