ooh shared water DWC, first come first served in the water cycle, as UK420Show pertained to, the bigger healthier plant is taking more than one slice of the pie, maybe try an foliar spray on the struggler🤔
All plants are unique even if they are the same strain they will not grow the same :) one may need more of something than the other and vice versa I have grew 2 plants in the same pot before and found they were both fighting for the nutirents meaning it was a never ending battle trying to keep them both happy but it just didnt work so yeah safe to say I didnt do that again lol