“Don’t pass food directly between chopsticks.”
“This action is reserved for handling bones after cremation. The family passes the bones of the person between each other to the container that holds it. Some see it as a bad omen to pass things between chopsticks otherwise – Chinese and Japanese usually. It is just generally rude in any setting unless designated otherwise to be rude.”
While there a sanitary byproduct to this tradition (no saliva exchange as etiquette demands that when picking up food from the center of the table, the person picking it up should use the backside of the chopsticks – this is true for also then serving that food to other people at the table), this action is primarily something set aside for interacting with the dead. As with the etiquette to not stick chopsticks upright into anything – particular rice – food and food related tools have specific designations when interacting with the dead vs the living.