A laptop may not be capable to supply power to all USB devices if power hungry even if laptop is AC connected. There is a limited power amount plus the actual USB circuitry could possibility fail. On top of this you want two(2) GPS units installed, seems unique to me but also be the issue in setup, to include the GPS power demands.
In the control panel, review devices installed. If multiple yet *same* device is present, delete them and reboot and allow PC to detect, then reset them.
I am having problems with USB's that require configuration to run devices. It started with a wireless mouse that worked with fine just with Vista but was not working after I configured the USB outlets for a GPS device. A second GPS device that also required configuration by CD caused the continued failure of the mouse and the failure of the first GPS device to communicate with the laptop.
My laptop has four USB ports and I hoped that there is a solution such as assigning each device to a separate port. Is any solution possible?