Looks like nute burn to me. Do you double the nutes because there are two plants in the same pot? This is one of the many good reasons to use the "one plant, one pot" technique, lol.

Are you adding any suppliments, like CalMag Plus, Superthrive, any micronutes...?

Stinky gives very solid advise. I too would go with the flushes on a schedule, and perhaps cut-back (a weaker solution, same schedule) on the ferts for a bit.
Likely, if this one needs a flush, the others will need a flush soon, too. How are the leaves actually looking on the others? Any signs of burning at all?