Vista - Multiple scanners

I have both an Epson desktop scanner (Perfection 1640) and a Nikon film
scanner (Coolscan IV). Both are connected via USB. Only one can be
recognized (under Imaging Devices in Device Manager) at a time. I have to
turn off one scanning device before the other can be recognized and used.
This is a minor annoyance that was also encountered with Windows XP, but I
had hoped that it would be resolved by now (Vista time).

Back on Windows XP I remember that the conflict also occurred when I was
using a USB-connected Visioneer Strobe scanner, so it seems that it is
generic to all scanners.

Seemingly Windows Vista is limited to only one USB-attached scanning device
at a time, but this seems like a rather arbitrary and unhelpful limitation.
I don't know if the limitation is from Windows or from the USB subsystem, but
I have never seen any mention of it in Windows documentation or media
articles.

Can anyone shed light on the source of this annoyance?

Thanks.