I don't think you can do this. It sounds like you are connecting your laptop to an output port of your USB hub. I think you will have to unplug the usb cable from your desktop and plug it into the laptop when you want to use it with that.
This isn't very elegant of course and networking the PC and laptop to share the printer/scanner would be a better solution. I'm not sure that the scanner would work over a network but the printer would be ok. To share a printer on a network you right-click the printer and choose sharing.