Humberside Police said bogus callers were putting in orders "specifically to steal vehicles, with the likely intention to commit more crime."
At 8.15pm on Monday a a silver Vauxhall Corsa was stolen from Green Lane in Cottingham as the driver went to the door to make the delivery.
On October 16 a silver Toyota Avensis was stolen from Woodlands Road in Hull.
Two cars had also had their number plates stolen.
Detective Inspector Allison Sweeting said, “We are investigating two car thefts in Cottingham and Hull where the fast food delivery drivers had left their cars running as they went to the door to make their delivery. I want to warn other drivers to be aware that bogus callers are ordering food specifically to steal vehicles, with the likely intention to commit more crime.
“There have also been two thefts of number plates from cars, which again will be used to put onto other cars to commit crime.
“I’d like to hear from anyone who knows about these incidents so that we can catch those responsible.”