I've heard of gouramis darkening depending on whether they are spawning or wanting to mate. I believe I've also heard of them darkening for no reason and then clearing up.
But if it is just on the fins of two of them...I'm not sure. Do you notice any of the fins missing or shredded in appearance? If not, it probably isn't fin rot.
Does it look like they have been dusted with a bronzish powder across their backs?
At any rate, I might add some melafix and up the salt content a wee bit.
Good luck
Hopefully its nothing!