Just f*ck them more than three times each, wait for a couple of days then go to your living room and wait until they are both there to trigger the start of the harem. Make sure their love and lesbian points are maximum or very high otherwise something bad will happen
Just found out that in order for the d*ck icon to appear, you need to f*ck them via booty call and not a date. It will appear when you booty call them successfully more than three times each . It usually appears when you interact with any of them in almost any part of the house.