Beth and Anna were 25 year old women who had known each other for their entire lives. And for their entire lives, they absolutely despised each other. They were rivals all throughout high school, competing against each other for better grades, and competing for the same guys to be their boyfriends.
They were unfortunate enough to unknowingly pick the same college as the other, and they ended up majoring in the same category: engineering. Because of this, they ended up taking mostly the same classes as each other, continuing the rivalry they waged during high school.
Why did they hate each other? Well, they won't admit, but they were jealous of each other. Both of them had good looks and great bodies, both of them were very intelligent, and both of them were particularly well off financially. They were jealous of the others good fortune, causing them to always want to outdo the other.
Even if they actually liked each other, they had different interests as well. Beth was more of an indoor person, and would rather read a good book or play some chess instead of going outside and playing soccer. Anna, on the other hand, loved sports. She worked out often, and she was very good at basketball, tennis, and swimming.
So basically, neither of them wanted to have to interact with the other for long periods of time. Unfortunately for them, that exact situation they wanted to avoid would be taken to the extreme when they were abducted by aliens.
Beth was in her house reading a book, of course, when a UFO landed right outside her house. When she looked out the window to investigate, an alien stepped out of the UFO, and used mind control on Beth to cause her to immediately fall unconscious.
Anna was also at home, watching the popular TV show Breaking Bad. She was getting towards the end of season 3, and was about to finish one of the final episodes of the season, when a probe from the same UFO was launched, and shot through the roof of her house, and directly into the room she was watching TV in.
She screamed in terror as the same alien from earlier, when Beth was abducted, stepped out and forced her to drop unconscious. The alien dragged her to the prone, and they both went into it. The probe was lifted from the ground by the UFO with a magnetic field, and the probe was brought back into the ship.
Once both Beth and Anna were in the ship, they were placed on operating tables side by side in a room full of different equipment capable of performing many medical tasks. This equipment, obviously, was highly advanced.
The girls were strapped down to their operating tables, and both of the tables were flipped so that both of the women were facing forward. Each table was rolled into a separate pod from the other. On the opposite side from the room, there was another pod, except it was empty.
As you may have guessed, the pods the girls were placed in were activated. These pods were used to teleport objects to different locations on the aircraft quickly, coming out of the other pod. The aliens must've theorized that if two objects were teleported to a singular pod at the same time, the objects would be combined into one.
This theory would hold true, as when they aliens decided to teleport both of them into the pod on the opposite side of the room, only one body was left strapped onto the table. The clothes on the women's bodies unexpectedly disintegrated, which left a nude body on the table. This body, however, was anything but normal. This body was made up of Anna's left side, and Beth's right side. There was only one head on this body as well. Oddly, the left half of the face was Anna's, and the right half Beth's.
Before the women even woke up, the aliens could only assume that their minds were combined into one consciousness.
Now, instead of deciding to observe them together on the UFO, the aliens decided to drop them back into Beth's house, and would instead observe the women from afar.
When they woke up...