The T-Rex Chocolate Extinction dessert (along with the Rainforest Cafe counterpart) are somewhat anomalous in terms of the dining plan.
For most table services places, the standard dining plan meal includes an entree/main course, dessert, and standard drink. When using the Deluxe Dining plan, an appetizer is also included.
A handful of places have items that say "for 2" or something along those lines on the menu. In that case, it means that two people on the dining plan must both agree to order that item.
I guess the easiest way to think of this is that each diner has one entree/main course entitlement, one dessert entitlement, one standard drink entitlement, and (if on the Deluxe Dining plan) one appetizer entitlements. The entitlements can be combined to order the larger items.
For example, let's say there are three of you going to Via Napoli (person A, B, and C). You are all on the Deluxe Dining Plan. So that means you basically have three appetizer entitlements. If you wanted to order the Fritto Misto platter, it takes two appetizer entitlements, which would leave one appetizer entitlement available to order something else. So you could order the Fritto Misto platter plus one other appetizer or you could order three individual serving appetizers.
Now, for T-Rex and the Chocolate Extinction (ice cream and brownies), it's not clear how many people have to pool their dessert entitlements to order the item. It's been reported that it required a minimum of 3, 4, or more individuals to order it when using the dining plan. It would be great if T-Rex would have a standard requirement, but it doesn't seem like they do.
I would basically go with the expectation that if you order the Chocolate Extinction, it will be the only dessert your group will be able to order using DDP credits (you can always pay OOP for additional desserts). If you are dining solo or with only one other person, there is a good chance that they will not let you order the Chocolate Extinction.