Not saying I know the answer, but in Mo. my ballot had a front and back side. When finished, I fed it into the machine one time-2 was not necessary. According to articles I have read, Mo. also used dominion machines.
“When finished, I fed it into the machine one time-2 was not necessary.”
You are correct, optical readers are on BOTH sides of the scan and the ballot only needs to be fed into the machine once. That’s why there was a big SNAFU about some polling stations being advised that sharpies could be used - because sharpies could cause bleed-through of the ballot and potentially create errors or spoiled ballots.