Police were called to the address in Hazel Way, in Crawley Down, West Sussex, at 10.18am where they also found a man seriously injured inside, police said.
He was taken by ambulance to the Royal Sussex County Hospital, Brighton.
A man has been arrested on suspicion of murder, confirmed police.
Detective Chief Inspector Alex Geldart, of the Sussex and Surrey major crime team, is leading an investigation into the incident.
She said: “At this time we believe this to be an isolated incident, with no ongoing risk to members of the public.
“This is a fast-moving investigation which will see significant police resources deployed to the scene for the forseeable future.
“We are grateful for the support and patience of the local community while we conduct our enquiries.”
DCI Geldart added: “ My thoughts are very much with the friends and family of the two women who have sadly lost their lives.”
Anyone with information about the incident is asked to report online or call 101, quoting Operation Deanland.
Alternatively, please contact the independent charity Crimestoppers anonymously on 0800 555 111.