With the new multi-target references (#1509), references in your language can now target multiple elements at once:

interface Obj {
    A: number
}
interface Obj {
    B: string
}
// `Obj` is a reference to both interfaces (not only one of them)
// Our type system should merge those definitions
const x: Obj = {
    A: 123,
    B: "hello world"
}

There are a lot of existing programming languages out there that provide features like interface merging or partial classes, which allow you to target multiple AST elements with one reference.
This feature is now available in the grammar language via the [+ReferenceType] syntax:

Item: name=ID;
ReferenceElement: item=[+Item]; // Can reference multiple `Item` elements with the same name

This will generate a item: MultiReference<Item> field now.