Extract from http://support.microsoft.com/kb/310123

Code 42
Windows cannot load the device driver for this hardware because there is a duplicate device already running in the system. (Code 42)

Recommended resolution

A duplicate device was detected. This error occurs when a bus driver incorrectly creates two identically named children (bus driver error), or when a device with a serial number is discovered in a new location before it is removed from the old location.

Restart the computer.

On the General Properties tab of the device, click Troubleshoot to start the Troubleshooting Wizard.

What you could try is to use a different type of USB-port for the two disks. One on the back, one on the front. Or one on the mobo or one on a PCI-USB card. That might be different enough to be not the same.
If that doesn't work, try exchanging one of the two for a different make/model if you prefer to have these disks external. Internal, of course, is much faster, so you must have a good reason to use them as external disks.

Hope this helps.