You may have suffocated the roots on the yellow one from overwatering. I would transplant it very carefully into the mix that you made for the other one. You may notice that the roots have turned brown when you look while transplanting. The stuff you put in the other soil aerates and loosens the soil. was the original soil just dirt? Or a potting mix? Sphagnum peat moss can get acidic from overwatering, so always water less than you think you should. I'm an oldtimer and always watered from the bottom into the tray, then poured off any water left after 2 minutes. Roots grow down towards the water supply as they do in nature. Rain water accumulates deep in the soil , not on the top. If it rained every day plants would eventually drown, they live off the water down under. Also the soil additives HOLD water, so let them get pretty dry between waterings. TOO MUCH LOVE KILLS, this is a weed, not some magic plant. If your sick one dies, take some cuttings and put them under the dome when the healthy one dies. Being a girl, maybe you're not to proud or stubborn to take my advice. PLEASE buy a Marijuana Grow Book and be very careful about taking advice from anyone on these sites, as I see SO MUCH MISINFORMATION given here, because so many people really don't know much. They think cuz their plant grew that they are experts and they SPEW WIVESTALES which are not from any actual scientific knowledge. I'm not claiming to be an expert, but 25yrs growing outside taught me a lot. The most important thing was to READ what the EXPERTS TEACH, like ED ROSENTHAL'S books, not what some kid says here
GOOD LUCK