So they are One?
Maybe there's an answer to this, but time and time again they appear to have separate consciousness, time and time again being surprised at the actions of each other, and time and time again those actions being a detriment to the other. It's hilarious that Elisabeth keeps taking the substance yet gains no benefit from it at all.
If it was clearly shown that she is in control of Sue and all her actions and is not SURPRISED by her actions, then the substance may be worth it if done exactly as instructed.