first off all you need more than 1 Db sessions and then you have to do this
copy ur diablo3 folder to somewhere else, change its name , go to ur diablo3 copied folder and go to diablo III.exe, right click and make shortcut, now go to shortcut and right click on it, go to properties and where u have like "D:\lolz\diablo3,exe" -launch (add: -launch at the and) and now u can run multiple diablo3 instances