Gerry Farrell is the expert on this burial ground but I think it would be safe to say that gravestones were not allowed on common ground in any Scottish graveyard or cemetery. There will be more than the 2 you mentioned in that grave, probably 2 more but none of them related, so one family would not be allowed to erect a stone for one (or in your case 2) of the paupers buried there. Don't forget, if someone was buried in common ground it meant that the family couldn't afford to bury them, let alone erect a stone.