Answer
Rather than trying to build separate autofill logic to populate the second field, define it as a Read Only custom attribute instead of a persistent one, using a Select or Expression implementation that looks up the corresponding name from the selected ID value. Since the underlying ID is already being stored via the first attribute, there's no need to separately persist the derived name — a read-only lookup field automatically reflects the correct value based on whatever ID is currently selected, achieving the same practical 'autofill' result without dedicated event/trigger logic.