Either because God decided you needed two, or because of evolution (in which case, you may eventually have 3 or 4)
my questions:
what color is a chameleon?
why does the register at work announce "scale failed" in a loud voice only
after I have hit the button to reset the scale? By then I've not only pretty much figured out that the scale failed, but also solved the problem. I asked my boss this question, but he just smiled and walked away.