Patrick Bentley made the pleas Tuesday in Mason County Circuit Court in Ludington.
Under a plea deal with prosecutors, Bentley is to get a prison sentence of 48 to 67 years. Sentencing is scheduled for Dec. 18.
Nineteen-year-old co-defendant Lashawn Whipple pleaded guilty in September to second-degree murder and other charges. She's expected to be sentenced to 30-60 years in prison on Dec. 4.
The bodies of 18-year-old Lacey Bentley and 35-year-old Fernando Ramirez were found Jan. 27 in Victory Township, near Scottville. The suspects were arrested later in Kentucky.