I voted early in DeKalb County, GA. After my ballot was printed, I checked it then went to the scanning machines, there were two of them. They asked me for the first letter that was printed at the bottom of the ballot and sent me to the scanner on the left, even though the scanner on the right was not being used. They did the same to the person in front of me.
My question: why would it matter which scanner a person uses?
In answer to your question
If you have more than one precinct voting at a location the scanner must be programmed to read ballot for each district. Down ballots are often different. Their ore the location of vote varies. Local races (school board- county offices etc vary radically within a county. . All races must appear on one page so font size and spacing often vary Thus two different machines may be needed to read different precincts voting at one location.
IMO this is why so many ballots only voted for Biden. They couldn’t line up the remaining contests to the various voting machines on the fake ballots So it was designed to read only one. The first one.