Very simple, if your game is already installed and you need the second client without interfering with your current client, run your " regedit " as administrator...
Open HKEY_LOCAL_MACHINE > SOFTWARE > Blizzard Entertainment > World of Warcraft
You will see 2 entries named GamePath and InstallPath - Delete them. They are left over by the current client version when installing but they are not needed after, and will hinder your chances to install a second client.
Once that's done, close your regedit and find the installer for enGB or enUS or whatever client you want to install and save it to a seperate folder than your original WoW client.
This helps for situations such as enUS alpha client being released and enGB players cannot play them.
I hope you enjoyed my guide, thanks for reading.