Well from what I can gather there is a possible risk that you accidentally drop a little blood on the wrong spot when getting it.  I know I find it hard to be steady when Ashley milks his arm and we’re trying to get so much blood.  It would be easy to contaminate one of two and have to start all over infact.  Also I guess it’s because they have had mix ups between which spot was used at what time.  it’s confusing enough as it is.  There is also the risk since the spot are so close together that one will be leeached into another?  I guess particles of dried blood could mix also?  The metabollic nurse did say to save mix ups with time the spot was taken and when--even in the lab when they are snipping them out.  Guess its not that hard.