Windows 7 problem with 2 portable hard disk same capacity

I bought 2 portable hard disk wd elements se portable 500 gb, the big problem is that when I connect both of them to my computer windows 7 recognize only one so I can’t use them in same time, I tryed to change letter unit of one of them but desn’t work, how I can fix this problem?

It’s a Windows thing see if this helps see post 11