I'm running both of my externals over a USB hub, and the hub and drives all have their own AC power also. I've never encountered a problem like this...
Do both drives work properly when one or the other is the only one plugged in and on? I'd see about that first, then try plugging them in one at a time both with and without the USB hub. Could even be a problem with a USB data cable so I'd swap them and see what happens when you try using them one-at-a-time after that too (to check if the unrecognized one changes over to that drive with the other drive's cable).
Passing those tests, then perhaps the drives themselves are simply conflicting. Apparently some of them do this, especially if they're different brands or one is new and the other old. A fix for that is potentially (and I say potentially because it's never happened to me and I've never had to do this personally, just know about it)...
Make sure both externals are powered on and plugged in, then:
Start > Search "Computer Management" > open Computer Management. Under "Storage" in left-pane of the Computer Management window (click the tree arrow if its not open already), enter Disk Management.
Now, in the Disk Management window, if you see both of your external drives are in there (recognized by windows), and one of them says "Offline" you will need to right-click on that drive and select "Go Online".
And that SHOULD do it, if this is the issue.
I'm spent for ideas after that, hope someone else can help you if I didn't.
EDIT: Actually, are both externals formatted to NTFS?