German rally organisers said they had cancelled the last stage of the day.
"Two Dutch participants died in an accident on Saturday for yet unknown reasons at the historic demonstration race 'Classic'. Despite immediate rescue efforts the two men died on the spot," Trier police said in a statement.
Police said the accident took place at the Arena Panzerplatte, a former tank training facility that includes the notoriously tricky "Gina" jump, which catapults cars some 40 metres.
The accident happened after the World Rally Championship cars had passed, German rally organisers said after cancelling special stage 14.
"The organisers of ADAC Rallye Deutschland have cancelled stage 14, the repeat of Arena Panzerplatte, due to an incident in a demonstration event after the passage of the WRC competitors," organisers said in a statement.
"Panzerplatte was scheduled to be the final stage today and WRC competitors have been re-routed back to the service park in Trier."
Jari-Matti Latvala lost the lead of Rally Germany after crashing out. Thierry Neuville has therefore been promoted to the lead of the event, where he now has a slender advantage of just 1.1s over Citroen's Dani Sordo.