For this you need to leave both Jill and Rebecca behind, and then not fly off into the sunset at the end...
To get this, just after you enter the room using the Spencer mansion key (to obtain the metal object) you'll get a cut scene where you'll hear Rebecca scream, simply ignore it and carry on to the courtyard.
For Jill just don't bother collecting the MO discs, then you can't unlock the jail room in the lab.
just to note you wont have to battle the Tyrant on the roof with this ending either.