Complete disaster. Nutes lockout, root rot?

[pogmek]

I'll go with the rules first, then explain carefully the situation

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Strain - Honey Cream
# of Plants - 2
Grow Type - Soil
Grow Stage - Flowering
Bucket Size - 12.5 liters
Lights - (1) 250w LED
Nutrients - Biobizz grow, bloom, top max
Medium - Atami light mix
PH - 6.5
RH - 40 - 55
Room Temperature - 25°
Room Square Footage - 40*40cm
Pests - ???

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That's what happened:

There are TWO of them growing in the same pot, same strain (50/50 indica/sativa).
Since the first day of sprouting, there were very visible differences: the one on the left had shorter internodes, big leaves, typical indica strain characteristic, the other one (on the right) grew taller with thinner leaves.
Everything was going incredibly well until the flowering begun, the plant on the left stretched a lot more than the right one, its' leaves began yellowing really soon from the tips, all over, while the one on the right thrived as green as ever.

A week later, the sick one didn't show any sign of growth, and the right one began yellowing just like her sick sister did.
A week later again things got out of hand, the left one displayed those rusted marks and splotches, while the right one begun losing more and more green.
As of now, the left one hasn't grown in weeks, and I think the right one might be stunted for ever as well now.

What I've diagnosed:

I'm almost sure there's been one time when I've overwatered / overfed her... as both of them dislpayed burnt tips (presumably nutes burn)... I've flushed since then and tuned down the nuted but it doesn't seem like it has helped.
What might have been a nute lockout, I think has transformed in root rot, but I wouldn't know how to check for sure.

What do you think has really happened, what do you see here?
I gave up already on teh fact that I'll yeld terribly this season, and that's fine cause It'll help me not make the same mistaks again... But what now?
I have to end their suffering and kill them, should I do it as soon as I can or at least wait until all fan leaves turn completely yellow? What's the "less-worse"?

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I want to thank everyone who will try and help me... and I'm sorry I have to let you see these poor girls' suffering. It hurts.

 

[Virgin Ground]

Two plants in one pot complicates things.Root rot stinks. Is there a nasty funk you can detect? They could be locked out , just hungry or root bound. Some deficiencies look a lot like nute burn in the early stages.

Do you have a way to test soil pH? Failing that ,can you test your run-off?

This will get you started troubleshooting, until someone more experienced can chime in.

 

[pogmek]

Two plants in one pot complicates things.Root rot stinks. Is there a nasty funk you can detect? They could be locked out , just hungry or root bound. Some deficiencies look a lot like nute burn in the early stages.

Do you have a way to test soil pH? Failing that ,can you test your run-off?

This will get you started troubleshooting, until someone more experienced can chime in.

Thanks for your reply... I doubt it's root rot at this point, i can't smell anything funny (last time I flushed, I took care to smell it closely and it was just wet dirt-like)... sadly I can't check the Ph... there's not much we can do from here I suppose... I would just like to know if I should keep flushing, or feed them so they don't keep eating their leaves up

EDIT: It's the second time I grow two of them in the same pot, first time went amazingly, this time... I learned I won't do it ever again