So they are One?

They are definitely not of one mind/consciousness. If they were, the copies wouldn't be so blithely careless about sucking the life out of the originals. Elisabeth, Sue and the old man all spoke of their clones as she/he, and Elisabeth even said "I don't know what she was thinking" when speaking to the customer service guy. They are only one in the sense of being genetic clones, or of depending on each other for survival, but that's about it.

Elisabeth might have needed Sue's income to maintain her own lifestyle if she had been living beyond her means and had little savings for retirement. There wasn't any indication of that in the movie though. What was apparent was that Elisabeth had started living vicariously through Sue. Especially after she had been turned into a total hag, that was all she had left.

They were seperate, the only reason the "company" kept telling her that is becuase the company was malevolent. Who in thier sane morally correct sound mind would offer anyone such a nightmarish drug withholding the most important consequences? The company probably derived pleasure from her suffering. Im sure the model male nurse got some kind of kick-back for recruiting new victims.

In my mind, and this is just my mind.. When Sue came into existence / was born.. she was another being. Totally different human with her own consciousness and mind. But Elizabeth was "in" there. Elizabeth was seeing/hearing everything, but Sue was in control. As far as when Elizabeth was in control.. I don't think Sue saw any of that. Sue didn't give a flying fuck about Elizabeth. But Elizabeth was enjoying being "in" / "there" when Sue was living the life Elizabeth wanted.

This is what resulted in this gigantic clash between the two.

yeah, it really doesn't make sense when you think about it after the film. What's the benefit? Elisabeth only gets to live half of her normal life and would also feel like she was getting older twice as fast. I wonder if it'd break the movie if they'd made Elisabeth's consciousness own Sue's body, she'd still be increasingly tempted to spend more time as Sue, while abusing Elisabeth's body for nutrients. It may have been even more of an interesting acting challenge to have Margaret Qualley copy Demi Moore's mannerisms.

They are not 'one'. And that's why the Substance supplier keeps insisting them to believe 'they are one'. But it can never work out. When two persons are infact different, how so hard they try, can't think of one and eventually would end up in differences in respecting each other.

The customers become Jekyll and Hyde, but the Substance's manufacturer tricks them into believing they are only 1.

Well... they are one in the sense that they cannot survive without each other... So you can say they are not one mind or one body... but they are one life.

From what I gathered, and I may be completely wrong and probably am.. I think all of the people that have taken the substance immediately create another person. I'm gonna keep mentioning Elizabeth / Sue names here so as not to confuse the two.

When Elizabeth brought Sue into existence using the substance.. Sue was another being, with her own mind, body, and soul. A completely different human with her own consciousness and thoughts. But Elizabeth was "in" there. Elizabeth was seeing/hearing/remembering everything Sue was doing, and although Elizabeth had no control over what Sue was doing, Elizabeth was still enjoying living vicariously through Sue, as a sort of "#2" / "tag-along". Eventually Elizabeth started to hate/resent Sue, but was still loving tagging along.

When Elizabeth was in control.. I don't think Sue was able to "see" or "tag along" with any of that life. After all, Sue was the "other self" (That's what it said on the food bag, remember - Food for your "other self" and food for the "matrix" - Matrix being Elizabeth). Sue was just asleep/in stasis when Elizabeth was in the matrix - ie running the show. Sue didn't give flying rat's ass about Elizabeth. Sue kept on sucking out Elizabeth's "life juice" without a care in the world. Elizabeth kind of hated Sue... but at the same time... was enjoying being "in" / "there" when Sue was living the life Elizabeth wanted, which created this huge internal battle within Elizabeth. Elizabeth knew Sue was destroying Elizabeth's body, and that bothered Elizabeth a LOT, but at the same time, Elizabeth loved seeing Sue's life and felt like Elizabeth was a part of it, enjoying being young and pretty and the star of the show and really looking forward to that whole New Year's event. Elizabeth had this gigantic internal battle with herself the whole time. That's why Elizabeth couldn't decide on whether or not to terminate Sue the very last second.

One of the main things that made me think Sue was her own consciousness from the start - Remember the old man in the restaurant - the one that first told Elizabeth about the substance. He was also referring to Sue as "she" which means that's the way all of the other selves are from the substance. They are different beings. The only one that kept saying "you are one in the same" was the guy on the phone in charge of the substance. The guy on the phone was wrong!

Or, maybe I'm wrong.