Corinne Cripps, 29, of High Road, Ickenham , and Maxine Benson, 32, of no fixed abode, entered their pleas at the Old Bailey on Monday (February 1).
Police officers were called to High Road, Ickenham, at 5.35pm following reports of a stabbing on Monday, November 9.
The London Ambulance Service were called and the victim, 18-year-old Alfie Stone , was treated at the scene before he later died at a west London hospital.
The two women also denied charges of wounding with intent to cause serious injury to Jake Stone, and Benson pleaded not guilty to possession of a knife.
A third defendant, Steve Hawgood, 28, of Kingston Avenue, West Drayton , has not yet formally entered his plea for charges of murder and wounding with intent.
All three defendants were remanded in custody and their case has been scheduled for trial on Monday April 4.