Why is One growing perfect the other is struggling

You got some nutrients burn going on the tips, need to cut back on the feed likely half what you're feeding them now, and yes the deep color is a sign of nitrogen toxicity I would flush but you also might be locked out on the light one as well do to ph, looks like you're running coco and clay balls and if that so I'm my buddy who is a veteran growers with autopots and coco told me before I started with my autopots that clay balls create ph fluctuations in the coco and aren't really that great for that system...You need to be a experienced grower to pull off the clay and coco to keep up on the ph fluctuations...Also my first time in coco was a disaster because I didn't buffer my coco properly with Calmag and I didn't ph the coco property from the start, you could definitely use a flush and I would try to determine what the ph of the coco is before you do so and adjust the flush accordingly...and I would cut back on the nutrients for a bit as well...I switched my autopots over to living soil, worm castings and Gaia ammendments for less stress and hassle...Good luck hope they turn around, just make sure you're ph is on and give them them some calmag to after your flush add it to reservoir