That dark green color looks like it could be getting a bit too much N. With your other plant yellowing, is it an overall yellowing, or is it more of the bottom leafs? If bottom leafs with yellowing inbetween greenish veins, it could be Mg deficiency... or it could be both...
Also, are your plants the same strains? Dif plants eat at dif rates...
I'm just starting in soil and coco in the last year after doing hydroponics for many years... I have to say, I think hydro is easier, from the standpoint of knowing EXACTLY what's in my nutrient solution. Coco seems to be a bit of a hybrid of soil and hydro. Gotta love growing!