Windows cannot detect all of My Passport Wireless Pro 3TB

I have 2 My Passport Wireless Pro 3TB and my windows detect only 1 drive when I plug both of my drive to the laptop at the same time. So, I think normally it should detect 2 drive but why it detect only one. How to solve this problem?

PS. I use windows 8.1

Windows 10 also?

Did the detected 3TB drive show normal size in Windows Explorer? How did you connect the undetected drive to the laptop? How about re-plugging?