Your best bet is to rename the second installation and try that.

you see changing the name of your first installation makes an invalid pointer so lets you install.

when the client install looks at where the registry says it's installed it sees the folder and says play not install, when the registry points to an invalid file then it lets you install again.

Your registry will still say TBC is installed into where you originally installed it which is where your second installation is now installed, changing the name of the second should let you install, and if it doesn't you can easily delete the TBC registry file which will allow you to install then. (always backup registry files before you edit or delete them)