The dual USB connectors are meant for those laptops which don't deliver as much power via the USB port as would be necessary to power the item correctly - quite a lot of USB devices come with them now, like external Harddrives and other USB powrred goodies. Worth trying one to see if the extra power helps coverage in weak signal areas. They are available separately via ebay and online retailers.
Laptops like the 12" Mac Powerbook need the dual connectiors to power hard drives - so worth trying the dongle on one.
Laurie