(For two or more people) to eat together from the same plate; (for two babies) to suck from a woman's breast (not necessarily at the same time).
The -an imp. focuses on the one who is to be joined in eating, the pa--an/pa--un imp. focuses on the one who is allowed to join s.o. in eating.
That married couple isn't comfortable eating unless they eat from the same plate.
I didn't join him in eating from the same plate.
Don't join him in eating from the same plate for he has tuberculosis.