Oh that Easy follow my step by step guide and you can get the scene with only Lexy.
Step 1: In night 2 when the power is out choose the “search for a vent or another way out to the room” (little note you can choose whatever to do in the vent it won’t matter).
Step 2: In night 3 after replacing the camera in the repair bay “tell zero to go back in her pod” then “restock soda wand” then “Clean up blood”
Step 3: In Night 4 do “Restock energy bars” then “vacuum booths”
After you done all three steps when you begin night 5 Lexy the shark will be unlocked.
The next night in the daytime lexy will make herself at home in jack black apartment and around there you will trigger the sex scene with lexy. (I think???)