A. Vertical and horizontal lines have very different causes. The problem can be defective electronics in the scanner, and these lines often appear as red, green or blue lines. The problem can also be dust inside the scanner, on the CCD or mirrors, blocking light from reaching the CCD, and these tend to be dark lines, and relatively few in number. The problem has also been reported to be caused to be installation problems for parallel port models too, but it never worked better in that case. Vertical lines tend to always be at the same place on the scanner bed, affecting specific CCD cells.
Horizontal lines are an event in time.
These lines are more random, showing at the location of the carriage at that point in time. In any event, lines are not caused by your scanning procedure, and calling your scanners Customer Support service seems a good plan. Generally no, not with desktop scanners. It is not a very practical idea. Two users cannot use the scanner at the same time, not in the way disks are shared, or even how printers queue jobs.