Usually ballots are 1, 2, 3, or 4 columns. Looking "through" the paper, a left justified oval on the front will be on the "right" of that column on the back. You have to use mixed columns or multi-column candidates for ovals to match back-to-back.
If you are worried about matched ovals, choose layout options that make matching impossible.
Having said that, I find it hard to believe that for visible-light readers we can't find a writing instrument that:
Should we hire an office supply consultant perhaps? :-/
Ken