Any alterations to the first drive are logged at time. By using the second, they can que up a bunch fo changes at once, insert them at one time when its less suspicious on the log, and no one is the wiser if they only examine the logs off the first drive. Example:
Vote goes on, you get some precincts in and want to add 100 votes for your guy. You que up several of those votes onto the second drive. While doing so, you find out your guy is going to lose by 150, so you que up 100 more. Now with 200 votes on the second drive, you wait for precinct 2 to come in which you'd win in normally, insert the 200 votes into it to make up for the loss in precinct 1. No visual discrepancy at all, no odd log for precinct 1. Only one log in instead of 2, at a time of your choosing, in a location of your choosing.