I happened to ask this at Prime Time on May 31. I was not on the dining plan but asked about the dining plan because I was curious.
The reply was that only two people on the (deluxe, because only the deluxe gets appetizers)dining plan can order the onion rings for two. One person cannot order them and pay with one credit. It has to be shared between two guests on the dining plan and counts as both their appetizers.
There have been other posters who said they were allowed to have appetizers for two without having to share them, but none of those was specifically at Prime Time and I was not trying to get the onion rings on the deluxe plan, just asking. It can't hurt to ask but don't be surprise if they tell you it has to be two diners ordering if you're on the deluxe dining plan.
However, if you are paying for the onion rings, one person can order them. The restriction applies only to the dining plan.