Step 1: Open up a game client normally
Step 2: Go to the folder in which you have installed Jade Dynasty, in the element folder find ElementClient.
(the file should be in a folder something like C:\Program Files\Perfect World Entertainment\Jade Dynasty\element)
Step 3: By double clicking you should be able to open up another client without any trouble.
You can open as many clients as you want as long as your computer can handle it.
Please note that using multi-client is an offence. Use with caution.