Nesh,
You stated it was fine after repotting. Did it grow after? Roots the same as bro? Was he repotted at the same time?
I was worried about contaminated soil but that weed is ok. So?

Since it is likely dead. I would pull it out, to literally, get to the bottom of this.
My guess is root rot. But that healthy one has the answers. Consider all their differences.
"right next to" could have been directly under a downspout that poured into only him.
Watering 2 times a week sounds like leaving it wet a bit too long. That soil looks like it would stay wet for 2 weeks.
I would use something that dries in a day or less.
Please share more info.
Sorce