The arrests happened around 2:00 p.m. Thursday on southbound Delaware Route 1 near Middletown Odessa Road.
Delaware State Police say a trooper saw their Toyota Corolla following another car too closely before abruptly changing lanes without a signal.
The trooper pulled the car over and, while talking to the driver - identified as 35-year-old Javona Smith of Chesapeake, Virginia - he allegedly smelled burnt marijuana.
Police say the car was a rental vehicle not authorized to leave Virginia.
Troopers searched the vehicle and found a plastic bag inside a jacket sitting in the trunk.
The bag contained 170 grams of cocaine and 515 grams of heroin with a potential street price of more than $250,000, police said.
Smith and her passenger, 40-year-old Sherita Nicks of New York City, were arrested and charged with two counts of possession of a controlled substance and two counts of conspiracy.
Both were behind held on secured bail.
Troopers say the smell of burnt marijuana came from Smith smoking a joint shortly before driving.