Kelsey Turner and Jon Kennison, accused of killing 71-year-old California psychiatrist Dr. Thomas Burchard, made an appearance on Thursday in a Las Vegas courtroom.
The former Maxim and Playboy Italia model had a relationship with the doctor for several years before his death.
Turner's body was found on March 7 in a car in the desert outside of Las Vegas.
Both Turner and Kennison entered not guilty pleas. Their next court appearance is June 27.